I've been using a bumper, but I don't have a skin (they're coming in the mail). I think the Bumper is ok, but I much prefer something protecting the back. I'm eagerly waiting for my Pixelskin HD from the freecase program, but the bumper is better than nothing. I'd feel much better about it with a skin.
The thing with a bumper is that, I keep my phone in my front pocket, and without a back on the case, say if I run into something about waist high, or if I fall on that side in some strange situation, there's nothing to protect the glass from a dead on hit. I'd much prefer a back to my case