I missed it tonight, but I've seen Matt Smith in a number of other things. To be honest, I'm kind of worried whether a 26 year old actor can pull off the role of the Doctor, but Matt Smith has been excellent in everything I've seen him in and has a face and attitude which belies his young age. So although I think we have to wait to see how he does as the Doctor, I have a feeling a lot of people will be pleasantly surprised.
Above all, I trust Steven Moffat's judgement. If he reckons Matt Smith is the best actor for the role of the 11th Doctor, I would be very surprised if he's wrong. Who can doubt the man who wrote stories as brilliant as "Blink", "The Empty Child / The Doctor Dances", "Girl in the Fireplace" and "Silence in the Library / Forest of the Dead".
To those above who think it won't be the same - I thought that when William Hartnell regenerated into Patrick Troughton, but I'm still watching after 45 years. Of course it won't be the same, but it will still be worth watching and just possibly might even be better. That's the beauty of Doctor Who, changes such as this keep it fresh and interesting.
In the words of the 6th Doctor just after his regeneration "change my dear, and it seems not a moment too soon!"