The first few seasons (series for you wacky brits among us) are pretty good, but IMO the show really started falling apart around when Torchwood started and I haven't watched it in years.

The things to remember is that it is among the softest of Soft SF on TV, that the Doctor can and will do anything if the script requires it no matter how absurd, and that continuity (both in the literal 'time' sense and the narrative one) is completely b0rked. Your enjoyment will likely come from the performances of the actors and quality of the production rather than from the writing, and if that isn't enough you'll likely drop out as the actors get worse and worse and the plots get more and more self-indulgent over its run.