Short version:
every bad thing i've heard about this inexpensive 2AAA headlamp is true. but it is brighter than i expected it to be and the low mode is bright enough for hiking on easy to moderate trails. the battery life on the low mode is pretty good and there's no sign of PWM on the low mode. tint and beam shape are excellent.
Longer version:
the Princeton Tec Byte is maybe one of the worst reviewed headlamp on the internet. you'll find mostly negative reviews of it almost everywhere you look. the complaints revolve around three main issues - poor runtimes, high parasitic drain, and poor durability especially regarding the battery door. if there was ever a headlamp that i wasn't going to buy, it was the Byte... until i recently found one for less than $5 still new in the box. then i was like, whatever, i'll try it.
i got mine and yup, the battery door looks and feels incredibly flimsy. and of course it's designed so that if the door fails the lamp fails. i popped some Eneloops in there and yup, the high only lasts for 2 hours before cutting out completely, and that is very short when the output is only rated at 35 lumens. (i haven't tested parasitic drain.)
the thing that did surprise me was that the high looked noticeably brighter than i expected it to be. i compared it side by side with a ZL H60 Med1 which is rated at 45 lumens OTF and, after accounting for the difference in beam profile, the Byte's high looked about as bright. compared to a Fenix E05 (27 lumens ANSI), the Byte's high was definitely more than just 8 lumens brighter. i compared the Byte's low to the ZL's Med2 (15 lumens OTF) and the Byte's low was brighter there too.
i'd guess that the actual output of the Byte is about 45 lumens for High and 20 lumens for low. the 20 lumen low means that the low mode is actually bright enough to hike moderate trails with. this is kind of important because it means that the Byte can actually be an effective "get you home" light when an afternoon hike goes long and pushes into the night. the low mode runs for about 9 hours, which means that the Byte can also be an effective light for dusk to dawn, all night hikes if you ever do one of those, without needing a battery change.
and the low mode doesn't show any signs of PWM. this is good for me because low frequency PWM often makes me dizzy.
also, the beam's tint and shape are great for outdoor use. my guess is that the tint is right around 5500K; it's very close to day light neutral. and the beam shape is similar to an XML P60: 60 degrees wide spill, wide hotspot, relatively soft edge between hotspot and spill. it's great for hiking and pretty good for general home use too.
i would not use this for backpacking. i like to only take proven reliable gear on multi-day trips where every malfunction gets magnified. the battery door design issue is just too much for that. combine that with the high parasitic drain reported by others and i would not use this as an emergency backup or a headlamp to keep in the car either. i would not use the Byte in any situation where reliability and durability are major concerns. that also means it's probably not good for kids.
but if you need a light weight headlamp (mine weighed in at 60g fully loaded) that is small, comfortable, has a good beam, if you have a use in mind for it where reliability isn't critical, and if you can find one for around $10, it's really not that bad.
man, that's a lot of ifs.
Note: Princeton Tec usually offers a lifetime warranty in the US and 10 year warranty internationally for most of their products. the warranty on the Byte is for 5 years for both the US and internationally.