I think that, after the Matrix movies and Constantine, Keanu Reeves desperately needs a movie in which he can show us that he can act -- if he can act. He is perfect for that look of bemused idiocy he wore all through the Matrix movies, but it was certainly misplaced in this movie. Luckily, Rachel Weisz did act, and did a terrific job of it too.
Go see it if you have an afternoon to spend, but it's certainly not a must-see.
And if you do go to see it, stick around during the credits. Afterwards, there's an extra scene that sort-of adds to the movie.