This is such a great idea: high-capacity Powerbook batteries.
I had two stock Apple batteries. I was careless; I never maintained them. Now, both batteries together don’t last an hour. They’ve gone bipolar. That is, they need more lithium. (a little DSM-IV joke for you therapists in the audience)
My new 4800mAh battery is being exercised right now: spinning a CD through iTunes, on eternal repeat with full-screen visuals. The fan is kickin’. My old batteries couldn’t do this even if wired in series. So far, I’m impressed.