You can't help but to simultaneously like and loathe each racer for his manner. What's more, they clearly have that same the same sort of like/loathe aspect to their rivalry and quasi-friendship. It's a nice trick, and it lets a viewer slip into the main characters' heads just a little. And the cars and action are fun to watch.
Bottom line: 3.5/4 stars. You can read other folks' opinions over at Rotten Tomatoes, but they largely concur.
As a final note, Gravity is out and by all accounts it's great. I'll be seeing it very soon. But it is sucking up all of the cinematic oxygen right now, and as a result Rush may not linger much longer in theaters. Get yourself down to your best nearby theater and see it on the big screen, before you miss the grand spectacle and are left with nothing more than a Blu-Ray disk and lingering regret.