iBook battery
I bought a white iBook in 2000. It was the 500Mhz model with OS 9.1 & 10.0.1 at the time.
When I first got it, the battery lasted almost 5 hours (as Apple claims); I was getting around 4 hours per full charge. Slowly over the 2 last 2 years the battery became weaker and weaker. It got down to about 30 minutes and the battery would die at around 70%.
I shelled out ¥16000 (in Japan) for a new battery just about a month ago, and I'm back in business again

I'm not getting 4 hours, but I can comfortably use it for about 2-3 hours and still have at least around 40% left.
The iBook is by far the best computer I've ever had (I like it better than my TiBook) and I really wanted to keep it working, so for me it was worth it to shell out for a new battery. Of course, I'm up to OS 10.2.5 now, and 576Megs RAM, so system usage can get pretty intense, and it can get pretty hot at times. I'm not sure if this machine was even designed to run the latest version of Jaguar, so that most likely could have an adverse effect on battery life.
I make sure the CD drive is empty, and turn the brightness down and mute the volume and it lasts for a long time.
If you really want to keep it for the future, you might want to take the plunge and buy a new battery. It helped me.
-5300cs