yeah, applecare is amazing (at least from the exchanges i've had with them!).
i needed my powerbook's screen replaced, but we don't have a store anywhere near us. called applecare and found out i still fell under the "send a box" warranty. was thrilled about that, but still figured it'd take a week at least between all of the shipping and actual repairs. i was wrong! between the time i called them and the time the return box was on my doorstep, only three days had lapsed..!
Applecare doesn't cover "wear and tear" meaning eventually, your battery won't hold the same charge it once did, and if/when that happens, you've just got to shell out for a new battery.
AppleCare covers defective batteries. Those that seem shoddy in the first place. Given how new the MBPs are, that battery shouldn't have been dying so soon, meaning it was defective, which would be why Apple replaced it for the OP.