Batteries are expensive. I typically prefer to use rechargeable ones when the purpose is sensible. In some cases, one outweighs the other.
In my wireless PC keyboard, it doesn't make sense to use rechargeables, because the keyboard only uses about two sets a year, and would be useless during the 2hr recharge period. However, in my camera flash, I use rechargeables. The flash eats four AA's for breakfast every day, which would be outrageous for Alkalines. Also, an added bonus, the flash recycles faster with rechargeables. If you've ever waited on a flash to recycle and missed a moment, switch to rechargeables to save some money and save some time.
When it comes to rechargeable AA's, I've tried a couple of companies, including Canon and GP. The Canon's run about $5 each and the GP's are about $3.75. To be honest, they're both great. However, if I was to go out and by a few more today, I would be buying Sanyo Eneloops, for sure. I have heard nothing but praise after praise about these guys.
I bought the GP's four at a time, and bought a separate 8-bay charger. The Canon's and Eneloop's each come in four packs with chargers, or you can buy just a four pack "refill". For my flash, I got a four pack, and a refill pack. That way, I don't need two chargers for my eight batteries. Four are charging, four are flashing. Then switch. It's a simple effective system.
Aside from the AA's for a moment, my Panasonic uses it's own rechargeables packs. The camera ships with one "medium" sized pack, and my used camera came with an additional "large" pack. My goal was to have three large packs. At about 5hrs each, that would get me through an extreme 15hr day. Which never happens, but that's the point. I already had one large, so I searched for two more. Panasonic sells theirs for $140. I have no interest in dropping $140 to have Panasonic stamp their name on the back, which I would cover with Gaff Tape. As much as it's frowned upon, there are hundreds of generic battery packs all over eBay. I went there. I didn't run out and get the cheapest, but I wasn't paying $140. I found a small battery store that had generic large Panasonic packs for $40. Bingo. I bought two, and the company got them here in three days. Great!
In my use so far, these things are rocking. Rocking. I will be sticking with them, but will make sure and report any grief they give me, if they do. I took some (more) Gaff and numbered the packs with to keep them straight. I charged each one up, drained it all the way, charged it up and drained it. Now I've got them all charged and "reset" (at least I feel like it anyways) and ready for action. My handwriting doesn't look anything like a lovely Panasonic logo would, but I would hate staring at it knowing how expensive that label cost.