I loved series 1. The 9th Doctor was great (he's my favourite, and it will take some doing to knock him off the top spot), the stories were a good mix of entertaining, thought provoking and scary, there were all those tie in websites, good stuff. Rose was good too (she became intensely annoying in series 2, but I liked her in 1), and the Bad Wolf arc did work quite well, I'm less convinced by later ones. Admittedly the Slitheen would have been more effective without the farting at the very least, but I presume that was put there to distract children from the horror aspect of killing people then using their skins as disguises. Dalek was brilliant, as was the Empty Child/the Doctor Dances and Father's Day, I think it was the best series so far. I rather liked Boom Town too (I know most didnt). The balance of everything was just right. My only real concerns are that the TV shows referenced in 'Bad Wolf' means it won't age well, and that the Doctor had to go and regenerate at the end.
Series 2 wasn't awful, I watched it and there were many things I did enjoy- mostly School Reunion, which was great fun, but also some aspects of the Cybermen episodes- Rose the dog, converting alt-Jackie was quite effective, and the handbags in the finale were funny. But I hated the smug couple thing with Rose and the Doctor, she was supremely irritating. I found the 10th Doctor too happy, bouncy and hyperactive, and a bit annoying (he improved though), the Torchwood 'arc' wasn't much of one and didn't really contribute to the ending. I was disappointed Rose wasn't killed off, as we'd been told she died, and would have found the series far more effective if she had been- not just because I disliked her, but it seemed a bit too much of a get out clause. Also thought it should have ended with the scene either side of the wall in 2 dimensions, but that's a minor point. Overall, this series does not stand up to the others, had far fewer good points than any other and far more annoying ones. Which isn't to say it was bad, just not as good as the rest.
Series 3 was an improvement, though I was disappointed in Martha- her crush on the Doctor seemed a waste of her potential as a companion. Though I quite liked her apart from that. It had some really good stories- Human Nature/the Family of Blood, Blink, Utopia and the Sound of Drums (but not the finale) were my favourites, I liked seeing both interpretations of the Master, though was very disappointed by the resolution of the series. Also didn't like the Doctor pining for Rose, it was very annoying. Not as good as series 1 overall, but the second half of series 3 was very strong.
Series 4 was better again, and in my opinion the best 10th Doctor series. Donna was much improved from the Runaway Bride (she had started to improve even by the end of that episode to be fair) and her story was brilliant and utterly heartbreaking. The balance between comedy and more thought provoking stories was excellent. Planet of the Ood and the Fires of Pompeii were great both for Donna's character development and for making the audience think, and I loved the friends dynamic between the Doctor and Donna. Donna was a more independent companion and more willing to question the Doctor than the previous two which worked very well. I liked Silence in the Library/Forest of the Dead, while the vashta nerada weren't wonderful, the basic premise of the story was interesting, the young girl and Dr Moon both very well played, and the cliffhanger was a good one. River Song was interesting and played as an equal to the Doctor, despite being human. I actually thought the resolution to that story, though presented in a positive light, was really quite horrible, I don't like the idea at all. Midnight was good, great idea- not sure it quite hit the mark it was aiming for, but still, very good. Turn Left was brilliant. Didn't think much of the finale though, all the companions, too much to fit in and an over convenient resolution to the threat- while Donna's importance, and that of the 'spare hand' had been well signposted, the actual Dalek control box was just silly. Caan's threat that 'one of them will die' meant another get out clause- though in this case I'd call Donna's exit worse than being killed off- and I'm afraid I found that 'towing the Earth' thing utterly ridiculous. Still, the final few minutes worked well.
So far I'd say the series were excellent, ok-ish, pretty good and very good respectively. I await the next series, and new Doctor, with interest.