Do It!
A few months ago, I noticed my battery performance had declined a significant amount. I set up an appointment, went to the apple store the next day, and ten minutes after i walked in, i was walking out with a new battery for my MBP. No credit cards, no crazy questions, and no paperwork. I even had a great chat with the guy who was helping me. All of this, despite the fact that I had already had my first battery replaced on apple's dime during the regular battery warranty period.
A friend of mine bought a MBP and his battery also started to show poor performance, after the warranty period. He went to the same store, but since he didnt have applecare, he had to pay for his new battery.
Having apple care has already half paid for itself, and knowing how many moving parts a computer has, and how a laptop is something you take everywhere, i'm certain something else could happen, for which i will likely not have to pay a dime to have fixed.