I have not read the novel but it is on my list. So I don't know how true the film is to the Oscar Wilde story. (Considering a good portion of the film takes place well after Wilde died, I'm pretty sure a lot of that was never in the novel.) I do know that Dorian Gray is a cautionary tale about the soul and the consequences of your actions (at least I think it is). And given the subject matter, it would have to be a fairly melancholic piece to begin with. What the film makers did was take that dark story and make it even darker. Gray is portrayed as a man really without a mind of his own. He is easily influenced and of a very weak character. This makes it hard to really have any vested interest in Gray at all. Instead, I found myself more intrigued by the Basil and Harry characters that are Gray's influencers. This is especially true of Harry because he's the driving force behind Gray's depravity. He's also played by Colin Firth who is (in my mind) the King of Hollywood right now.
It's odd really because Gray seems to be so empty and one dimensional. His only motivation is hedonism and to see how far he can take pleasure for his own satisfaction. It makes sense in a way because all of the consequences of his actions are deflected to the painting in his attic leaving him, ultimately empty. The big problem with this is that I feel it can work in a novel but that kind of character fails in a film. Without blatant (and thus, poorly written) exposition in dialogue, it's difficult to get into the head of a character like that. It can be explained with much more detail in the written word. As a result, on film, Gray just becomes this zombie of a sex addict and a lot like the characters in Crash (the sexy car accident one. Not the "everyone's a racist" one) and it becomes boring.
So, once you realize this, the only reason to sit through the whole thing is to see what the painting looks like. This is another symptom of the film maker's inkling that they may not have a masterpiece because they purposely leave out any decent shot of the painting after it starts transforming. You can call it mystery and suspense if you like but I just don't buy it.
All in all, it focuses too much on weird nudity and depravity and not enough on the underlying themes. Don't see it.