I use a Wave [http://www.griffintechnology.com/products/wave3g] on my 3GS, but only when I am going out somewhere where I am likely to drop it. Otherwise I don't bother with a case. The case comes with a screen protector but it's rubbish (you are guaranteed to get air bubbles due to the way it sits under the case), so I don't use it. Case seems sturdy and had no complaints about it effecting the finish of the phone (I do find it hard to believe that any case could do that, unless it was made of sandpaper!)
I did originally have a flip case which was nice and padded, but the flawed design meant it covered the speakers, so I lost about 70% of the phone's volume. I'd recommended the wave.
