The most recent one - 'Blink' - was indeed superb. I look forward to the end of the series where John Simm (see Life On Mars) will be making an appearance. And possibly a very significant one...
I'm a big fan of the original series (despite not being around when it was originally on) - even today - so I'm hardly judging it by standards which are that high, but I have to say that some of this series alone has been pitifully bad, though. From the top of my head:
'The Shakespeare Code' was, for my money, very poor, but just about avoided being dire.
'42' was cringeworthy.
The recent one in which the Doctor became briefly human was also a complete stinker, I felt (oh please, it had the co-ordination and writing of a very high-budget school play).
David Tennant's overall tenure as the Doctor certainly hasn't been bad so far though. There have been a few tearfully dreadful episodes, but not bad. Regarding Tennant as the Doctor himself, I think they could've done better (though also considerably worse). Ditto for his assistant.