I got a gelaskin (Cranes/Nanami Cowdroy) almost immediately after getting my iPhone a year and a half ago. And guess what? It's still in great shape! The artistic designs on gelaskins are fabulous and I get plenty of comments on my cover. J just purchased one for his blackberry and is also a big fan.
The only reason I won't give them 5 stars is that I've had trouble with their Gelascreens. These are meant to cover the surface of the iPhone, but are a drag to put on and won't stay down in the corners. Like most other screen protectors, they get air bubbles and pick up any dust particle that got in the way during application. But the screen coming off on the edges... grrrrr.
A little pricey, but my gelaskin has kept the back of my phone scratch-free and is still going strong so I can't complain!