Great Fun, Great Speed, OMG it's over...
Currently I am playing through a second time on a harder setting, since I have found the collectibles I am shooting through the game at "Light Speed". At this rate I would almost guess if you only count gameplay time, it is less than four hours.
Being a big Star Wars fan in general, as well as a fan of the first, it is sad to see them having so many problems (on both games). The initial Force Unleased had all the save problems, this one is short, and at several times, I have gotten stuck and couldn't move my character (one time while fighting Vader at the end, and that fight is long enough as it is without having to start over).
I would have to say rent this, as it will save you some money, and as short as the game is, it will definitely save you some time.