I saw Dredd yesterday and yes, the helmet never comes off. You don't see Urban's face in the entire movie. But that's okay, because Olivia Thirlby, who I hadn't seen since Juno, never wears a helmet and she's pretty hot.
I liked the movie. I thought it was pretty good, just enough story to put everything in motion and then it's balls-out action right to the end.
I did not see it in 3D and I don't think you need to in order to enjoy it. There were only a couple of parts during the movie where I thought "that would look cool in 3D," otherwise it seems like a waste. Besides, a lot of it takes place in the dark interior of a city block and I don't think 3D works well in those types of settings. Tron, for example, was mostly dark and I thought the 3D didn't pop like it would have in a brighter setting, which was a disappointment because I was thinking going in that if any film would look good in 3D it would be that one.