I think I'm approaching the end of act 2. As amazed as I was with the pacing in Act 1, it's like it just keeps getting better. The enemies keep ramping up their difficulty as I level, obviously, but the sense of your power growing is present throughout. One of my pet peeves with games is when the progression never feels like you're progressing. This avoids that stagnation in a major way, and I can't even put my finger on why.
From what I've heard, the pacing just keeps getting better.