AP's point about the cost of Li batteries is all too true. I've usually kept the one that comes with the camera and bought an unofficial but compatible one to use as backup, as they cost only £10-15. But the unofficial one I've got at the moment isn't entirely successful - it works but it doesn't tell the camera when it's running low, just suddenly cuts out.
All the same, as I said, the official ones have lasted 3-4 years without ever failing, and I think over that period they'd probably work out cheaper than AA batteries and be less fiddly/cumbersome.