The monster is one of the best new creations for the series! I mean, Vincent battling depression, and an invisible monster that only he can see who hurts everyone around him while he can't do anything about it until he gets help from some friends, and then it turns out that the monster doesn't want to hurt anyone, he's just blind, lonely and scared... That's great stuff. I know that most viewers don't watch Who as obsessively as I do, and most of them wouldn't even begin to try to find a deeper meaning behind a giant invisible space chicken in a series that also involves giant farting green aliens, rhino cops and angry pepper pots, but even without that subtext you're left with a very lovely, beautifully shot small-scale episode exploring the fragility of the human mind, perception of mental illness, and the beauty of art.
I'm not saying that Doctor Who should tackle depression that head-on every week, but I would love it if they would get a bit more poetic with their monsters once in a while. Reminds me of Douglas Addams, although he applied literal translations of literary/scientific/philosophical concepts more for comedic results rather than dramatic ones.
Matt Smith's best episode, and the best episode of Modern Who. Without a doubt. A very brave episode, and one that could have turned out very wrong very easily. But it worked. I'm kind of amazed/proud that the BBC allowed it to go out. I still remember that the original broadcast ended with a BBC voice-over telling viewers the number of the suicide hotline in case the contents of the episode upset them. I mean, that's completely insane when you think about it.
I understand the opposing viewpoint on everything regarding Doctor Who - which era is the best, if Pertwee is actually playing the Doctor or another character entirely, the great RTD-Moffat debate - but saying that Vincent and the Doctor is a bad episode is just objectively wrong.