Here's my 2 cents ...
I've had some good luck with NiMH rechargables. It is very important to get the correct charger for the batteries you choose to use, they are not as interchangeable as one might think.
How long they last in a wireless depends on their mAh rating ... bigger is better. You do have to make certain that large capacity batteries actually fit your mics. I have seen some that exceed the diameter spec for AA batteries and do not fit properly.
The other thing is that NiMH and Alkaline discharge at different rates so the battery meters in your units may not respond correctly.
wireless sales engineer
Line 6, Inc.