I'm watching the whole series on Netflix, and I'm in the second season, the first with David Tennant. Simply amazing. I really really liked Christopher Eccleston, and was quite disappointed when I found out he's in it only for a season. However right from the second series Christmas special, Tennant kicks all forms of ass.
I can't believe it took me so long to discover Doctor Who. Great series!
P.S. Billie Piper is hot.
Make sure to watch the old Who as well. It's a little campy, but the stories are great.