I personally started watching the show with Tennant, I think in his last season or two, and then of course the past couple of years with Smith. I've seen some of the 9th doctor and wasn't really impressed, but I've never seen much of the 'old' series.
Anyway... I love Smith, personally I do think he's better than Tennant. Honestly I think the whole series has been stepped up a significant notch since Tennant left as well, so it is actually kind of difficult to separate the variables and decide whether it's Smith I like or just the series on the whole but... I do like Matt Smith.
But, as alluded to, the whole series is just so... great, since they brought Smith in. The Doctor has so much more depth. Amy & Rory play off of the new Doctor perfectly and the entire chemistry of the cast is brilliant. The cinematography is so much better and the effects are as well, but still with that little bit of cheese necessary for it to be Doctor Who.
Of course what's really interesting is how the Doctor's evolved. Smith's Doctor is so much darker, much more violent, and much more troubled with his dichotomy of being a force for good and the savior of humanity while also being incredibly destructive and cold to.... a lot of other races. In fact, the Doctor has been getting increasingly mired in this and as a result Smith has had hardly the time to be the light-hearted whimsical guy Tennant was. He seems to fall on that side about as often as Tennant had his outbursts of anger and violence.
But luckily, Amy and Rory play off this perfectly. Rory brings a lot of comic relief and light heartedness that you miss from the Doctor's increasing moodiness. In this function, he's really pretty essential to the show, or it will get increasingly dark. Amy is a really fun, spunky character as well, and Karen Gillan is... really really hot. I'm really glad I don't have to stare at Billie Piper anymore. I'm really not looking forward at all to Rory and Amy getting replaced, I think they're two of the best parts of the new turn the series has taken since Tennant left.
Last night's episode was pretty cool, I'm excited for the new season.