It's good! The plot is kind of thin, but it's more of a character- and action-driven movie. On that level, the movie is a 100% success.
Some of the orbital mechanics is cheesy, so turn off your analytic mind on those parts and enjoy the special effects. Then there's a shuttle in orbit over two years after the last one landed, and some other space hardware figures in that hasn't been launched... Like I said, turn off your analytic mind and stop poking holes in the movie. It's an cheap easy game, but totally beside the point here.
See it in 3-D, at an IMAX theater if possible. It's that kind of immersive experience. Don't screw around and let this slide off the big screen while you think "Darnit, but I'll catch it on Blu-Ray." You will regret it if you let this one go.
Bottom line: 3.5 out of 4 stars, just for the sheer damn spectacle of the thing. (But Rush was slightly better, so if it's one or the other, go see Rush.)