I started with Ecclestone but my heart is forever David Tennant's. I both Matt Smith and Eccleston do a fine job though, it only took the first episode to convince me Matt Smith was a good Doctor (his I am awesome speech at the end of episode 1 pretty much sold me). He's mellower and a bit gentler but he does child like glee well and brings his own decent character to the table.
Still nothing can beat David Tennants ('O') face or his excitment or even his end of regeneration crazieness.
Matt Smith is helped by having by far the best assistants though. Rose was the first and pretty decent, Martha was meh and Donna was better than should have been, but then she should have been really bad. Amy and Rory on the other hand were memorable and fantastic and drove storylines forward and felt they had more role in the story than the get-kidnapped-scream-exposition generators that the others were a little bit. But on the other hand, his character is the whimsical recovery of David's angst, the happy person that was born afresh from his previous regenerations redemption, so he's got less to express.
I also think the writing is really good with Moffat (and the people complaining about time inconsistency, I mean we've had that in Doctor Who from pretty much day one, I'm surprised this is the first time people are recognising that there is not set way time works in Doctor Who. I mean have we forgotten Rose's Time Reaper monsters from right at the beginning (not that'd I'd be surprised if we hadn't tried to forget those)). I think whats bad about his reign, is the tone, which has shifted a lot. Moffat who feels a heck of a lot more professional, the directing is uniformly higher budget and more stylised, there are less completely bottom out episodes (like Dalek pig monsters and human daleks) although where we would have had a bottom out episode, we now just have a very boring episode (Pirates

). Also the stories are a lot more ...internety... Dinosaurs on a Spacehip, Robot Westerns, Pirates etc and someone pointed out there was more of a clean fairytale feel to the sometimes awkward more sci-fi of RTD.
So yeah Matt Smith does a good job, but also has had the least demanded of him so far. I want this to be his last season now, not because he's bad but I feel like it's a nice time to lay him to rest and try something new. As long as they develop his character in a new direction and he stops being the nice one, I'll forgive him if he does another two series, but after that it's time for someone new