I remember the Frost-Nixon interviews very well.
So perhaps I'm not the best audience for the film. Most of the film is very well written and involving, especially when it focuses on Frost.
Not so much on Nixon. Stone's film was certainly not great, but Hopkins created an interesting and deep character for Nixon without making him a caricature. Langella may have been passable on Broadway, but his Nixon impression on film is not even remotely convincing. And if you don't buy Nixon, you don't buy the film.
Even worse, it seems that the real story wasn't interesting enough, so the writers felt it was necessary to add imaginary scenes with Nixon. There's a lengthy phone call monologue in the film that is completely out of character with the Nixon in the rest of the film. Perhaps if they had portrayed Nixon as rip-roaring drunk (as in the opening of Stone's Nixon), it would have been a believable character change, but in this film, it breaks believability.
It was such a fascinating piece of history though, I recommend the film nonetheless, and then go rent the DVDs of the interviews.