I have one and I made the mistake of keeping it plugged in. After a while I was told that this was a no-no, but that warning came a bit too late. My battery doesn’t hold a charge for more than a half hour or so… and that really, really sucks, especially when I take my MBP to class. I’ve never replaced the battery, though. I usually just carry the plug around with me in my backpack and sit next to an outlet. Maybe I should get a new battery?