I have seen him play quite a bit. He was really great for the Celtics, too, but he would always bite on the pump fake. He's improved, for sure.
A lot of people think Marcus Smart is, at this very moment, in the top tier of defensive guards in the NBA. He averaged 2.9 spg and has a 7'9" wingspan. He's a ballhawk with good instincts. He also rebounds and boxes out better than most NBA PGs and many SGs.
Like Allen, he's best offensively when he uses his physicality and athleticism in transition/attack rather than rely on his outside shot. But TA has been around a long time. His shooting form is and has always been ugly. Also, Smart already has better ballhandling and passing skills than TA has, even though he really only started playing PG in college. Maybe Smart will never significantly improve his offensive game. We'll see. But I think it's a smart bet (no pun intended) to think he'll be at least a 33-35% 3-point shooter in the NBA. He needs to be coached up to know when and how to shoot. He's young. The sky is the limit.