I finally got my free black bumper for my iPhone 4 last night. I forced myself to keep it on to get a good feel for how well it works. Some thoughts:

• it doesn’t look as bad as I expected. Not as sleek as the bumperless phone, but not bad
• it’s less slippery in the hand since the bumper is a bit rubbery
• it is harder to get the phone in and out of my pocket
• it’s not as comfortable to hold and feels bulkier
• it fits fairly well, but once in a while there’s a little shift in position, which makes an otherwise solid phone feel less solid

Overall, I’d say it’s not worth it unless you really can’t avoid the “death grip”. The bumper’s coming off tonight.