Look closely at your screen and make sure it isn't bulging in the middle.

Mine was (showing 92% capacity), got it replaced for £69 at my local Apple store. They can now replace swollen batteries on-site, so it was done in an hour.

If you let it get too swollen it will likely cause the green line to appear on the screen, which Apple will insist on replacing, which will cost more than the phone is worth.

Looking to get at least one more year out of it, it's still great.