I replaced my stock powerbook battery recently with a NewerTech 53.3 Watt-Hour Battery, which is higher-capacity than the Apple originals, which are 46 Watt-Hour batteries.
My old battery was so spent by the time I finally gave in that it would give about 20 minutes on a full charge before forcing sleep. The battery warning would show up after about three minutes. So I can't compare the extended capacity directly to a fresh Apple battery, but the new one has been doing fine so far. I saw at least three hours from a charge yesterday, and it's letting me work at a coffee shop without worrying about finding a plug, and that's a nice change.