I leave my Mac plugged in perpetually, or nearly so. It's 53 months old and the battery is at 80% capacity. So instead of 7 hours I get 5.5 or whatever. It's not really noticeable yet.

Maybe I'm just lucky. I had to replace my wife's macbook pro battery a month ago, same age as mine. Her's swelled up though, and was preventing the trackpad from clicking.