I disagree. I bought 2 iBook batteries at the same time about 2 years ago. One battery stays in the iBook all the time and the other is my spare that I use for when I need to go away from power for long periods of time.
The battery that is in my iBook gets about 45 min to an hour. The other battery gets over 2 hours easily. I purchased a brand new battery and I get over 4 hours. I read something about the battery can be charged 500 times before it starts to lose it's charge. It's possible my primary battery has just been charged so many times.
Since this primary battery is already bad, I am just keeping it in the iBook all the time and have that extra power when moving the iBook to another location in the house. But I won't ever do that with any new battery.