Who? Well, that's the good thing, he's almost completely unknown, just like all the Doctors should be. Steven Moffat seems to love him. The only drawback is that he's astonishingly young, even younger than Peter Davison was when he took over, and I'm worried the BBC wanted a more kiddy-friendly face.
I'm sure he'll be fine though. I'm more worried about what iconic villains are going to turn up in Season Five, as there aren't really any decent ones left!