I watched Exum in his first summer league game, and in the game last night. After the first game, I was really impressed with his athleticism, quickness, court vision.
After the game last night, I was much less impressed. It was like the other players already figured him out, and he was far less effective. I have no idea what the box score may show, he might have turned in a great 4th quarter (which I missed). The point is this: I think it is way too early to say that Exum is going to be a better NBA player than Marcus Smart.
What I think distinguishes Smart from any player in the draft is this: he can have a crappy offensive game, and still be a huge difference in the outcome. His physicality is just unsurpassed. He is going to contribute much earlier than a lot of the other draft picks. I'm not in love with him yet, but I really like him, so far.