Saw this on Thanksgiving with my family, and quite enjoyed it.
I had checked out a few reviews on RottenTomatoes, and the common remarks that stuck with me were on the subject of Will Ferrell being "boring" as a boring IRS agent, and that the film was kind of a poor man's Charlie Kaufman.
I disagreed with the negative reaction to Will Ferrell--I thought he was great in a very reserved role. I was relieved never to see him go over the top, doing that "Will Ferrell thing." It made the character funnier and stronger, and I actually stopped seeing the comedy star Will Ferrell, and saw Harold Crick instead. Good on ya, Willy!
I will concede that the movie is a bit of Charlie Kaufman for the mass audience, but that doesn't necessarily make it bad. It's a simpler film with creative conceits that are more palatable for the casual moviegoer, but the naturalistic and restrained performances make it every bit as sophisticated.
I didn't care for Queen Latifah's basically useless character (she was only there for the frustrated writer Emma Thompson to have someone to express herself to,) but the others were played very subtly and engagingly. Dustin Hoffman lends a degree of believability to the fantastical plot with his calm, collected, supporting role, never threatening to overshadow the main story. I credit the assured directing hand of Marc Forster (he of "Finding Neverland.") I loved the way he depicted the life of the boring IRS agent in an appropriately slow yet intriguing way in the first half of the film. (My sister found it a bit dull, but I like that in a movie when it serves the story.)
Check out the movie, especially to see Ferrell in a restrained, appealing comedic role. I enjoyed it, and my somewhat less jaded companions enjoyed it even more.