I'd rate it a 7/10. I really like the settings, the surprise that you really CAN climb damn near anything, as promised. But it was the repetition that killed it for me. No variety in the missions, except for the very few plot assasinations. It just really dragged on and made some mistakes, such as having hundreds of "hidden items" and no real method of finding them easier...be it an audio cue like the pigeons on GTAIV, or marking found flags on the map like the diamonds in Far Cry 2.
It was still a really unique game. A collector should own it for sure. A casual gamer might not even be satisfied by a rental though, given the timesuck the game can be, and not in a good way.