Worth the extra money?
I've read that the original battery lasts for 6 hours. Is this correct? How much i can expect from the nupower in terms of battery life? What about Oncore batteries? My iBook specs are G4 1.20Ghz, 1.25Gb RAM and Leopard 10.5.8..
That's up to you whether or not it's worth it. Six hours seems a bit high for an iBook.
The battery I got from TechOrbits gave ~3 hours, 15 minutes with relatively heavy use, and has remained "healthy." If you need extended battery power and are willing to take the bet that the expensive one will give you this over time, then the decision is made for you. However, if after spending $86 on the battery you come to learn that you'd be fine with 3+ hours for twenty five bucks, then it likely isn't a good deal.
The thing is, for $86 you can get three quality batteries from TechOrbits (if that's what you decide to do with your money), without worrying about them burning out all at once. If you need the extended battery power, then you're going to be stuck spending ~$86 each time your battery dies.
Bear in mind that these are generally older items that are likely to wear out more quickly than those that are fresh off the assembly line.
Personally, there's no way in hell I'd spend more than $40 for a battery. But then, I rarely need more than an hour or two on battery power.