I think I've replaced the G3's battery at least once; I know I've replaced the battery on the IIsi we had before we got the G3.

As far as I know, the battery in my iMac, if there is one

confused

, has never gone out.
Some people lost their notebook batteries to some funky code in the 10.2.4 installation. It completely killed the batteries' vitalities, requiring replacements. That's one reason I like desktop machines better.
My old Palm IIIxe used to go through rechargeable batteries fairly quickly, which is the advantage of having them: just pop them in the charger for a while and they're good to go. I think I wore them out from extended use, though, as it got a much better lifespan from regular Duracels.
My Tungsten E depletes its battery fairly quickly as well, but I charge it almost every night, so whether or not that contributes to it, it will usually have a full charge by morning.
That's one thing that surprised me. I heard somewhere a long time ago that Palm PDA's could go for something like 2 weeks on a couple AAA's. I guess it depends on how much you use it, and whether it's one that tends to drain power more quickly (like my always-backlit Tungsten E).