I didn't really know what to expect when I walked into the theater. I had merely heard that the movie was good and that I should definitely make time to see it. I would make this very same recommendation.
King George V (you may recognize him as Dumbledore, or Michael Gambon) falls ill and eventually dies and now one of his two sons must take over the throne for him. Initially, King George VI's brother takes the throne, but later hands it over to KG6 (that's going to be his shortened name for the rest of this post).
For the first three-quarters of the movie, I couldn't understand why it was rated R. It wasn't a scary movie by any means, there was no nudity, no sex, hardly any language... then we reached the 3/4 mark and I figured out why. KG6's speech therapist realizes that he doesn't stammer when he swears, so he tells him to start swearing to keep the words coming out. So he does :) Really, it's more funny than anything else. I should note though that there is quite a bit of language for the rest of the movie.
I liked the movie because of it's superb cast, historical context (just before and in the beginning of World War II... I don't think I mentioned that anywhere above), humor, and excellent story-- history, I guess.
I highly recommend this movie!
P.S. Might I also mention that I love the simplicity of the movie poster?