The missions and characters are fun.
However, the game suffers from bad design, in that most of the sencond half missions are made on a "try again" basis. You woon't be able to complete them on the first try, because the game designers made damn sure of that. So between every mission, you have to go back home to save the game, then drive to whomever is giving you the mission. Cutscene (skippable, small mercies) for the mission, then drive to the mission, and fail. Try again.
So yeah, you will get many hours out of the game, most of them driving to and from the actual action, and trying the same missions over and over again.
Sure, some elements are fun, but I don't see why it is such a highly rated game. Saints' Row II was much, much better.