I haven't had a case on my SII the entire time I've had it and it still looks brand new. I've dropped it a couple of times with absolutely no marks. Samsung makes remarkably durable phones.
This is truth. My Captivate and SGS2 are in virtually like-new condition. People complain about the "plasticky" feel/build of Sammy's phones, but the fact is they use high-quality (and very light) plastics that make their phones more pocket-friendly and durable than most.