NiMH batteries under the sort of load that things like the Uniquefire AA-S1 put on a cell (2195mA measured) will perform way, way better than alkalines which won't give you the output. Or the runtime. It is pulling 2.6W from the cell and will do so for 69 minutes to 50% of original brightness with a 2000mAh Eneloop freshly charged.
It gave me 2830 lux at 1m for throw and 693lux in my lightbox. On a 14500 it gave me 6100 throw and 1535 total output - so it is more than twice as bright on a 14500.
These figures are not precise enough to be absolutes, but in comparison they are perfectly adequate.
I'll do this in tabular form where it will make more sense. Remind me... I'll need to do a runtime on an alkaline - I think I have 14500 runtimes somewhere.
Cheap rechargeables are usually not too bad as long as you don't go for high claimed capacities which are always overestimates (This is China we are talking about after all). Avoid anything with a claimed capacity of over 2100mAh altogether - it will be flat within a week of charging. The lower capacity cells (1700-1800mAh aren't usually so nasty and don't self discharge as quickly) But all the non LSD (low self-discharge) types will need to be charged within a month of charging even when not used.
Many of the 2500mAh+ claimed capacity ones are too physically large to fit in a lot of things and are very, very delicate. Some reputable "2700mAh" ones I bought a couple of years back never gave better than 2400mAh and wouldn't hold that charge for more than a couple of days.
Actually I'd only recommend the Sanyo Eneloops for NiMH cells. And I've been through a great number of rechargeable AAs over the years. Since I bought myself a stock of Eneloops I've not used the other ones much at all. Sanyo claim 1500 cycles for their latest (Japan only) version and 1000 cycles for the ones we can buy.
For a cheaper alternative, the GP LSD cells are well regarded. http://www.dealextreme.com/details.dx/sku.8746 They do them with a charger for $23. I bought some Vapex LSD cells. There's nothing really wrong with them, but for my purposes the Eneloops work better. The Eneloops cost almost 2x as much but were well worth it. In my very picky digital camera the only rechargeable AAs that work well are the Eneloops.
After 150 cycles in a digital camera, cheap NiMH cells are toast. Older digicams are very, very hard on their batteries. I never could persuade the person I share an office with to buy decent NiMH cells - she probably owned more of them than I do. By 250 cycles even if gently treated most cheap cells are going to be toast.
Chargers:
Completely avoid 15 and 30 minute chargers. They kill cells. Quickly. If you must use such things get 15-1800mAh cells of very good quality and expect to junk them annually. Even 1 hour chargers are pushing it. Slower chargers tend to be gentler though many people charge NiMH cells at high currents and then use them hot off the charger for maximum capacity. The RC model crowd expect to get very few cycles out of their cells but then they are very, very hard on them.
I own two Angeleyes chargers that I got from KD. Unless you want the battery analysis features these are OTT $35. Anyway, I can't find them on either KD or DX any more - both used to stock them.
For a straightforward charger that won't cook your cells at a reasonable price I'd go for this one. http://www.dealextreme.com/details.dx/sku.6152 $13.98
There are plenty cheaper ones but they tend to cook batteries. Heat is not good for any sort of cell/battery. I'd check the reports on any of them to see about their heating up batteries. Yes, they should be warm, but not too hot to touch - if they are, they have been damaged by the charger.
The Vanson BC1-HU is a decent charger, reasonably priced. It has been rebadged by many, many companies and is of distinctive appearance so is easily enough spotted.
The larger Maha chargers are excellent, but not cheap. Avoid LaCrosse chargers, they melt.
The upcoming NiZn cells and their chargers seem well worth watching but there aren't enough of them out in the field yet to see how they will bear up under hard use.
Apart from their environmental nastiness and generally low capacity, NiCd cells have a lot going for them. They will withstand incredible abuse and keep going. I own some NiCd D cells that were bought in 1984 and still provide above their nominal capacity though they won't give 20-40 amps on a dead short the way they once would. Their self-discharge is much lower than NiMH and they can be used to do things like start cars (I once did this with the 10 Ds mentioned above) - which NiMH cells might do once, they will be good for nothing afterwards. However nobody makes good ones any more. Cadmium is nasty stuff.