i got the Doubleshot and a screen protector (from HTC as well).
The Doubleshot Hard Shell is very nice, doesn't add much bulk. The grip is not as good as the original HTC back .. feel kinda' slippery. My only concert is that the plastic that covers the buttons will break, because the connection between the case and the button is very, very thin.
The screen protector does not cover the whole screen, but it's ok. I got that because 2 days after getting the phone I already had scratches. And believe me, when it comes to new gadgets, i'm very careful when using them.