DecalGirl.
Just installed it and it was worth every penny. It looks even better than I thought it was and the pieces are 99.9 % accurate. You can see just a sliver of antenna along the sides if you look closely, but I probably could have done a better job.
The DecalGirl comes with a piece for the home button, but I'm not so sure they intended for it to be used as the home button. I think they just didn't bother removing the cutout. Using it makes the home button kinda spongy.
It actually looks better in person, but I decided the front vinyl wasn't my taste so I removed it and only left the back and sides.
As for the reception problem, I was convinced it helped a lot, at first... I can always go from 5 bars to 1 holding the phone in my palm, but after installing the skin, I could only get it down to 4 bars. I was thrilled, but a second stab at it and I was able to lose all but one bar.
(I plan to copy and paste this in a couple other threads, word for word, but I don't work for DecalGirl. I have seen very little info on the iPhone 4 DecalGirl skins, so I thought I'd contribute by sharing my experience)