It absolutely wonderful. There are huge range in types of plastic. in some cases plastics are just as good, while being lighter, cheaper, easier to manufacture.
Of course it implies that plastic parts are designed properly, right plastic is used to manufacture them, and forces acting up on them are within range of what plastic can take.
I really dislike single-use plastic but have no objection to it for things that are intended to have a long life-span. As for lights, I tend to think that aluminium is probably the best material because of its thermal properties, especially with the high power available now with li-ion batteries.
I do like the idea of the light @extremesquared described though, when you consider that a 4 D-cell incandescent MagLite puts out about 100 lumens getting the same out of a plastic twisty light that takes a couple of AAs should be easy enough. Make it out of high quality plastic and it would be waterproof and almost indestructable, something like the Surefire polymer lights but single output and minus the tail-switch and aluminium bezel. You could probably also make it affordable. The problem is that I donāt know how much of a market there is for such a thing.
The only plastic light I currently own is a Photon micro light that Iāve had on my keys for over a decade, it still works and is still on its original battery. I think I would have to try really hard to break it.
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I think plastic is extremely good for many uses. Its cheap, its lightweight and its also a good insulator of heat and electricity which makes it good for many uses. Note that there are different types of plastic. Cheap stuff like PVC for kidās toys and stuff, acrylic for carās brake lights or plexiglass, but theres also strong plastic like HDPE for helmets and polycarbonate for safety glasses. Some plastic cant melt is thermosetting plastic like bakelite which is used in stuff where heat resistance or insulation of electricity is required like frying pan handles or housings for electric devices.
In terms of flashlights, I hate plastic flashlights because they are cheaply built and break too easy. There is also no heatsinking which limits us to small, weak low powered flashlights. Stuff like aluminium or other metals is superior.
For other uses plastic is great. Even packaging as the plastic can be recycled. It is melted and reshaped multiple times and there is less of a need to make new plastics.
I regret not picking up one of the GRP plastic solarforce lights in yellow when they were available for this exact reason.
I think the use of plastics for engine parts is the most impressive.
I think the main concern is about heat dissipation, as the metal body works like a heat sink.
Maybe for 1000- lumens it would be ok.
The key here, is āWhat for?ā
edit to add;
TL DR version;
inherent problem with plastics? Nope.
Prefer plastic? yes for some things.
Prefer other materials over plastic? yes, at times.
The long rambling answer I wrote first;
I have nothing inherently against plastics, For some things I prefer it. But whether I prefer or dislike any material, depends on, again, what for; I enjoy using all materials, for the right purposes or projects. Conversely, I hate some materials, even plastics, for some uses.
to address some plastic items that have been mentioned; I have no problem with plastic guns, I carry one 24/7 for self defense. Then again my favorite hunting rifle is 60 years old, blued steel and walnut. Iād like a new polymer stocked rifle for easy carry, and weather resistance, but its cost prohibitive, so I have no problem with a wood stock.
In tools, I love my modern cordless tools, plastic, light weight, and pretty strong. Iāve also seen them broken in the cold, or just generally on job sites. I also own tools from the 60s that are steel, aluminum, or even magnesium. I LOVE a chainsaw I have from the early 70s that is 90% cast magnesium. But my go to for hard heavy wood cutting is a 90% plastic unit. Its just so much lighter!
I prefer steel for my trucks(generally refuse to own anything made after 1980 so there is very little to worry about there), but I make plastic accessory parts for them too. But then, modern plastics used in autos are stronger, less UV reactive, and generally better than those in the 70sā¦ But even just asthetically I like Metal cab interior.
Since it was mentioned, I will say, about 20 years ago I got my first replacement radiator with plastic tanks; Hated the idea, but its lasted a Long time with no issues, and thats in one of the harshest winter enviroments on earth for half the year or more. I now donāt mind them at all, and actually prefer it because they cost around 1/3 as much (last I looked)as welded steel tanks.
In general plastic car parts (like fenders/ bumpers) are BAD here, they shatter in the cold winters. Interior parts crack and wear different in the cold too. Otherwise Iād probably not have an issue. Weāve had several very modern very plastic family cars that have had no issues with their plastic parts(other than broken bumpers).
To make other comparisons; Iām a hobbyist and semi professional welder, have a metal lathe, and soon a milling machine. I made knives for a living for a few years. I like metal a lot. I also do cabinetry and fine wood working as a hobby, and do construction, remodeling and flip houses for a living now. I do a lot with wood and like it too.
Then, I also own 2 3D printers, one acquired within the past week, and if I canāt make it in wood or metal for various reasons, I can usually make it (maybe or maybe not easily) in plastic, admittedly my main hobby currently(somewhat obsessively).
It can indeed be the superior material in a lot of things. But what plastic you are talking about has a lot to do with it too.
I have a hard time with " I hate plastic ITEM X, so never use anything plastic". There are about as many compositions, and grades of plastic as there are species of trees/wood.
Professionally I use PEX, PVC, CPVC, and ABS for plumbing. Different UV ratings, heat ratings, strength, stiffness, durability etc. And different legal and safety requirements too. You donāt mix them up.
Its the same with other materials too, like as said woods, there are right and wrong woods for some things.
Or aluminum gradesā¦ This is close to a rant for me, but Iāll throw it in here anyway; There is a big thing in consumer aluminum items, flashlights included, that say āAircraft grade aluminumā. for its strength and durability. Its marketing hype. Do you know how many grades of aluminum are certified or even designed for aircraft use? LOTS of them. How many of those are structural and Verry strong? Some of them. How may flashlights are made of those later grades? Iād bet its None in all actuallity, but many are made of the common grades that just happen to be used for aircraft, making the marketing point technically true.
Iāll shut up now, I have a feeling Iām off track slightly, or rambling on.
You did ask about feelings about plastics, not exactly about other materialsā¦ I just canāt answer without making comparisons. 
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There are uses where plastic should not be used, so it is not the case of plastic being a bad material. However even here plastic is gaining grounds, there are thermally conductive plastics, several manufactures make such plastic.
I did a little research about thermal conductivity in polymers and aluminium, and here is what iāve got:
-Thermal conductivity of aluminium 7075 alloy used in some flashlights: 173 W/m-k.
-Thermal conducivity of two thermal conductive polymers:
āCoolpoly: 34 W/m-k
āCrastin BK350: 14 W/m-k
Regular polymers:
HDPE: 0.4 W/m-k
ABS: 0.33 W/m-k
PP: 0.12 W/m-k
PE: 0.11 W/m-k
There is still a huge difference between them, and even with alluminium alloy bodies we struggle to have performance.
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But then there is titanium, which is around 22W/(mā K) IIRC. And it does work, even if performance is lower.
Actually usually performance is not limited by heat conductivity of material flashlight is made of, but by ability to dissipate heat into environment.
Both Pelican and Princeton Tec have been doing that for quite some time. Dive lights.
I guess I just want a low-output convoy s21 version of one of those.
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Thanks for looking up the numbers, not as good as AL, but then much much better than other plastics.
However, conductivity ratings for aluminum is for pure aluminum, which is never used, AL alloys have lower ratings, then we have anodizing, which also hinders convection, then we have star to heatsink junction, another loss, thermal paste even if applied perfectly still have lower than Al alloy ratings. most p60 drop ins have poorly finished surface where the star sits, thick paste, air gaps, brass pills,ā¦all these little things add up and really take a toll. So basically heat conduction index from led junction, to air will be very different than metal ratings.
Thermally conductive plastics are used in lighting fixtures, as long as they are properly designed/executed, leds are not driven hard, plastic is sufficient.
I donāt mind quality plastics but hate cheap plastics that become brittle over time. The problem is when buying something plastic is how do you know the difference ?
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If anyone was interested, these are the keychain flashlights that had the plastic crack and break:
Nitecore Tube
Nitecore TIP T Series XP-G2
TrustFire MiniX
Thatās every single keychain flashlight that I have owned that has plastic in the body. 
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You need a Photon. 
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Iāve got a bunch of Fauxtons somewhere.
I think I might have had one on my keychain, and it eventually broke.
Fauxtons are woefully dim, though. 
I still have few photons that I got about 20 years ago, changed batteries several times, threads are still intact. Same for free lights that battery junction used to give away with an order, blue ones, also about as old, and still intact.
As mentioned āplasticsā is a pretty broad term. The amount of everyday products that contain plastic of some formā¦
Also mentioned, in terms of flashlights āthermal and electrical conductivityā makes aluminium the default better choice.
So it really it comes down to use case scenario, pick the right material for the job.
How do i feel about plastics, fine. How do i feel about plastic based flashlights, they donāt really appeal to me as a general rule.
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Plastic can be good or bad based on a number of factors. Thermals tend to be bad, but not every flashlight needs to be made completely out of plastic nor run hot enough to demand a mass of metal heatsink.
Less concerned about the recyclability of flashlights (small, relatively durable goods) than other things made in far greater volumes with far shorter lifespans (i.e. packaging). Also suspect that the average metal flashlight is tossed end of life rather than recycled.
Suspect that as more materials become available for 3D printing methods other than FDM, plastic will become more hobbyist friendly for homebrew flashlight designs; conversely additive metal 3D printing looks to remain a more industrial process for some time due. Machining remains a bit of a niche, specialized hobby.
But for now the situation for hobbyist and small-time producer is similar. Injection-molded plastics require investment in tooling that machining aluminum does not. Conversely a decent aluminum design is on average mechanically more robust than most good plastic designs.
Another myth, that plastic is all recyclable, only about 10% or less can be recycled, so it has to be separated, Another shocker, is that there is no money in recycling plastic, the only reason it exists, cuz it is mandated by local gvmnt. NYC is heavy into recycling, but long island, at least Nassau county is not, there is no recycling law there, no money in it, so no one does there it.