I really enjoyed my Speck Candy Shell on my 3G and 3Gs.
I just scored my Speck iPhone4 Candy Shell today and it replaced my Apple bumper immediately. I did like the bumper but the phone just felt vulnerable. I swear I'd dropped the 3's 5-10 times total and they have been well protected.

I also opted for Zagg shields (first time) on the front and back of the phone.

It ends up like I'm carrying around this little black brick, but it's pretty bulletproof with the exception on face shots on protruding edges/corners.

www.speckproducts.com