Sherlock Holmes books; it was not based on a particular book, but it was very similar to their style.
I also thought that Robert Downey Jr. did very well portraying Holmes and that Jude Law made for a good Watson. I was getting tired of seeing versions of the Sherlock Holmes books where Watson was portrayed as a fat and dense sidekick, which was nothing like the books.
Overall, this was a very enjoyable film that is both fun and intriguing. The action is not overdone, the story keeps you interested, and the characters are believable.
As far as questionable content:
The movie stars Rachel McAdams. It seems to be a requirement to have her at least half-naked in one scene in every film she's in. The scene is brief riskiness.
The film also contains some dark scenes involving black magic that might be a bit much for some.
P.S. - The image used in this post is copyrighted by Warner Bros. Pictures.