I have a gel skin (case) from T-Mobile on my HTC Magic and it's alright. It doesn't fit as snug as I would like and it's a dust/lint magnet. Perhaps, that is the case with all gel skins though. Finally, the hole at the bottom covers up the microphone hole on the phone but ppl still hear me ok, so it doesn't seem to be a problem.
Overall, it does the job of protecting the phone.