I've got an iBook G4 12" (in sig) which I use almost daily still.
You *can* buy batteries from them, either very expensively from Apple or (as I did) from a cheap Chinese manufacturer on eBay. I get around 3hrs from mine, 1 year after replacing it.
Main problems:
Flash video decoding brings it to its knees, even though full screen h.264 decoding is fine. Flash and other apps are just not optimised for PPC.
In the same vein, browsing can be quite slow. Safari 4 and Firefox 3.5 do help quite a lot, but for long pages or Flash-heavy pages can be painful.
The screen is rather dim. My girlfriend's new Macbook Pro makes an absolute mockery side-by-side.
Try picking up a first-gen Macbook 13" if you can. I've seen them for as little as £350 on some sites.
Good luck, you won't be disappointed by an Intel Apple laptop.