At this point I have no opinion on him (except he should change his hair. Please.) How could I? I don't know him and I've never seen him act before.
I love Tennant and he was technically my first Doctor so its gonna be hard to say goodbye to him, but I have faith in Moffat. Plus people were dumbfounded when they heard Billie Piper or Catherine Tate were going to be companions - and look how popular they were!
The first time I saw Matt (hmmm, his name isn't as fun to type as Tennant) it was in the special Confidential episode and, to be honest, the first thing I though was "he's not as eloquent as Tennant is".
But thats really got nothing to do with it since he'll be acting off a script.
And then I thought "hahaha, he uses his hands to express meaning" which I guess isn't that uncommon but it sort of feels like a Doctory thing to do.
So yes, I can't form an opinion yet (very boring I know) but as long as he can convey the energy and humor as well as the age and wisdom of the Doctor then its all good!
Life the Universe and Combom did an utterly fantastic job of getting all the media frenzy, so if you want to read all about that, I suggest you see these two posts:
The question now is: What do you all think?