NiMH batteries have become the mainstay
in mobile consumer
electronics. The main reasons for this are:
They have a large capacity -as much as 2000mAh in a single AA cell.
They are light-weight. Again, an AA cell weighs about 40grams.
They are easy and safe to handle. Although the different kinds of lithium based technologies offer a higher energy density,
they have an annoying tendency to explode when mishandled(that's why Li-Ion batteries only come in intelligent packs that can take care of themselves).
It's Really Worth It(tm) to use NiMH batteries instead of alkaline -although you can get something like four or five alkalines for the price of one NiMH battery,
you have something in the vicinity of 800 charge cycles in one NiMH.
Over the last five years or so, NiMH capacities have been steadily on the rise. 2000mAh for an AA cell is the current maximum -expect to see larger capacities in the future.
To put this in the right context, alkaline batteries typically store 1500-3000mAh, but it's important to remember that rechargeables only drive 1.2 volts per cell where alkaline batteries are 1.5 and that means slightly less hours in a NiMH battery per charge.
Now, before you go out on a NiMH shopping spree, there is an important issue with NiMH batteries. This particular gremlin is called internal resistance.
All batteries have this. It's very low in NiCD and lead-acid batteries and notoriously high in NiMH and (in particular) lithium-technology cells.
The effect of a high internal resistance is that as you draw more current from a battery, the voltage at the terminals drops quite a bit. This means that NiMH batteries should mainly be used in electronic equipment that draws relatively small currents over longer periods. It's not that a handful of NiMH cells can't drive something that depletes them in about ten minutes reasonably.
The problem is that if you need a large current here and now (most likely for an electrical engine - and probably R/C uses), you just won't get it. This is of no concern to users who don't need large currents, but at least I've warned you now.