I like Matt Smith but have some problems with his performance. He mumbles a lot and his speech often lacks clarity - he puts emphasis on the wrong words or swallows words, and I am confused both as to what he is saying and what emotion he is trying to convey.
Visually he's fantastic, and he captures two things very well - the "old man in a young man's body" and the sense of amiable eccentricity. What he doesn't do, for me, is capture any of the Doctor's great sadness, loneliness, or at times darkness. The words have been there for him in all three of the episodes so far (the "I am the Doctor" speech in the first ep, the "nobody human speak to me today" in the second, and the confrontation with the daleks in the third) but those were his least convincing moments.
I enjoy the eccentricity of his performance but he's not making me feel anything. David Tennant caught the dark side of the doctor from the word go with the chilling way he said "no second chances - that's the kind of man I am", and he proceeded to show us that over and over again. He was very much an all rounder of a doctor, with great variations of shading in his performance. I'm not a DT fan particularly but I do think he's a very fine actor having now seen him a variety of different things, all of which he pulls off with a real sense of nuance and sheer acting talent.
I think Matt Smith could evolve. I still think he's been cast too young although I can see why the temptation was there to cast him because he looks so superb and gives such arresting visual performances. He needs much stronger direction though to bring out more layers in the character.