As mentioned in previous threads, I think Eccleston was a great transition from no Doctor to having a Doctor. Fairly Tom Baker-ish, comfortable and acceptable to old fans and new.
Tennant, though, is much better. It becomes more and more clear as the season progresses that he's truly a great Doctor, imo. Quite possibly my favorite, now.
I think jumping from no Doctor to Tennant would have been a tougher transition, though, because his personality is more "out there," and it's quite possible that older fans would have rejected him without Eccleston as an in-between.
All-in-all, I think it worked out great!