I have been using Varta (Max Tech) batteries in my flash units Canon 580 EX II (a beast of an external flash) which requires energy like no other equipment in the photography industry. There is a bewildering array of batteries available, but I have had no problem with Varta other than its price and the rate at which my flash unit was consuming. As good as they are, every set I bought was gone in a flash. I've been hearing everyone rave about the Eneloop batteries by Sanyo. They seem to be the standard in the DSLR community for testing flash recycle time, etc. So I ordered a set to see if they are as good as people say they are. I am not a battery expert but these are my observations:
1. Eneloop batteries are capable of providing the high current needed for fast recycling of the camera flash.
2. They maintain an even power curve while in use.
3. The batteries deliver more shots than any other batteries I have used before.
4. Unlike most rechargeable lithium batteries that take 7 hours to recharge, Eneloops NiMH, recharge in two hours or less.
5. The Eneloop brand has a longer storage life (can hold a charge for months, even years).
So unless you want to spend more time changing batteries than actually enjoying your cameras, Eneloops are the way to go.