Holy moly, what is wrong with some people here...what someone else said was right, this thread makes people paranoid...
I wouldn't replace a battery unless it was totally dead or not functioning or holding a charge.
People with anything less than 99% are talking about buying new batteries/acting like their dog died or something. Jeez. lol.
Anyway...I have a mid 2007 macbook that I recently purchased off ebay, and the battery capacity is at 91%. I am calibrating it right now. But overall I am impressed, not bad for a nearly 4 year old machine.
Edit: Hmm, since I've been calibrating it, the capacity seems to have gone up to 94%. Sweet.