Gravity is far and away the best hard-SF film I've ever seen. That doesn't make it the most fun I've had at the movies this year, but I can still tell when a film is actually good, and not just a good time.
There was one moment, just one, where the physics failed for me. I'll talk about it in the comments to avoid spoiling things for you. There may have been others, of course, but this film had me fully invested and totally sold so I only noticed the one. How did they film this without actually being in space? I have no idea.
I don't want to say much more than this. The film is a powerful ride, and the less you know about it the better the ride will be. I'll say this much, though: the soundtrack--not the musical OST, the film's audio component--took my breath away. Oh, and the musical bits were impressive enough that I bought them on iTunes before leaving the theater. But don't do that before seeing the movie. Track titles will give things away.
And do go see it. If you love science fiction, if you want science--more and harder--in your movies, this is one you don't want to miss, and it's one that you'll want to throw your support behind. As of this writing Gravity comes in at #7 for me for the year, easily clearing the Threshold of Awesome.