Like most people here, I have a fairly heavy bike: LHT, two panniers full of the usual anal commuter stuff. It probably weighs about 40-45 lbs total, heavy enough to be horrifying to a roadie, but not that bad for a commuter. And, like others have said, heavy isn't really that much slower in most urban situations than a lighter bike. However, if you have a heavy bike, you really need to watch out for your knees and IT band: if you charge up hills and take off from intersections as if you're on a Felt or something, your joints will suffer. It's best to pretend you're in a truck: start in a low gear, and gradually work your way up to full speed. And don't be shy about spinning up those steep hills in your granny gear at 6 mph.