I'll add to this too. I give my black TW case a little spray with my TV screen cleaner of all things, circular motion for cleaning, comes up like new every time. No sign of wear, or knocks, or scratches, or anything. Nice durable case.
My phone is/was also the same, until last night. I was walking down my stairs and it slipped out of my pocket, caseless: it bounced down 3 steps, hit the skirting board (scraping the paint off the skirting no less!), and hit the floor on its bottom corner and landed face down. It was Schrodingers broken iPhone for a few moments. However, I picked it up, inspected it thoroughly: the only damage it sustained is the *tiniest* pin prick of a mark: so small my wife couldn't see it (theres a joke there somewhere!). Don't get me wrong, it's not something I'd want to do again, but I feel a little more confident knowing that it took that kind of drop and strike and it came up ok. And, TBH, I can't even really see it today-you can't even feel the mark.