You have only ONE job!!!!!!!

Well, here's my opinion...

See, I saw the movie first. I knew there was a book, but figured I'd see the movie, then read the book. And honestly? I think the movie is better than the book.

Now, here's my reasoning.
While 'The Picture Of Dorian Gray' is one of my favourite books, without a doubt, it isn't perfect. Hell, it has many flaws. For example, the book is filled with melodrama. If you actually read it closely you'll see phrases like 'he threw himself onto the armchair and howled to God in pain and fury'... because they had forgotten to meet someone somewhere for dinner. Everything is over-the-top and overly dramatic, with every issue being treated as the single biggest issue of the time.
The film improved over this by simply having the characters act like real humans, not as over-the-top performers in a play. They don't leap around and shout every line, they actually act like real, believable people. If you don't think I'm making sense, take a good, long look at the book again, and you'll see it's crammed with melodrama.

Secondly, the writing is flawed by modern standards, in that it takes two pages of speaking to make a single point. I have the same issue with Mary Shelley's 'Frankenstein', in which the author would spend several pages describing how blue the sky is, or have a character talk for an hour about how sad they feel, without actually saying anything. It's not telling a story, or expressing an idea, it's just padding with words, as many words as are available.
What the film did to improve this was simplify everything. Too simple, perhaps, but intelligent enough that a mainstream audience can see it and go 'oh, I get it!'. Furthermore, a book is allowed to be as long as it wants, a movie has to fit into a certain amount of time, usually around 120 minutes. In order for this to happen, they really did have to simplify the story down to it's bare bones. They hit all the main points (the painting, Sibyl, deaths, Dorian's holiday, finale, conclusion) while still retaining the important elements from the book.

Thirdly, the ending wraps up the book in about five minutes. I honestly feel like Oscar Wilde had a better ending planned, but just didn't have time to write it, because the ending in the book is not only lack-luster, it's also unbelievably fast.
In the book, Dorian meets a young girl in a village that he falls in love with, and attempts to become a better person for her. But, in doing so, sees his portrait grows even worse every day, so he finds the only solution is self-destruction. He takes a blade and pierces the painting, dying as his old, rotten self, while the painting returns to it's former splendour.
In the movie, we get a far better, far more resilient ending, with Dorian falling for Lord Henry's daughter, Henry discovering Dorian's secret, a fight between the two, and finally (in my opinion the best element of the movie), Dorian literally facing his demons. Cap it all with a cold, hard line from Lord Henry ('Poor boy... who can bear to look at you now?'), and then end. A far better ending with a lot of closure, since the book wrapped up in minutes.

I feel like people get too caught up in the aura the book has and don't look at it critically enough. The same goes for things like Shakespeare and Dickens. We're too afraid of ramifications from our peers to actually give our honest opinion.
We praise works of fiction because everyone else praises them, while not having opinions of our own. I've read 'Dorian Gray' enough times to know that, while I still like it, it's far from perfect. Honestly, I'll take the film over the book any day, because in terms of exploring the characters, the story and the concepts... the film just did it better.
Yes, in my opinion, a movie from 2009 did something better than Oscar Wilde, a celebrated classical author. Shocking, isn't it?

I thought the movie had many flaws.

And was better than the book.

Wilde wrote one novel for a reason.

He spent so much time mastering the sentence he never found fluency in the paragraph.