Not realistic, not original, just fun as hell.
I don't want to say too much, because part of what makes this game great is the element of surprise. Almost every mission will have you doing something different (most often ridiculous or hilarious), and the whole story mode was enticing enough to keep me going and beat it in two days.
Customization is great. Weapons have a great upgrading system, or there are many other upgrades you can make throughout as you level up and make money. Character customization is great too, even if the graphics are a year or two behind.
The one complaint I have is that normal mode was pretty hard (at the beginning before i had any health or weapon upgrades) so I dropped down to Casual, but that got WAY too easy about half way through (but I kept playing in that mode so I could beat it quickly because it was a rental).
I do recommend just renting it however. The story mode is the majority of the entertainment. Sure there are a lot of great things to do in the open world, but most of these things you will come across before completing the story (so ask yourself is it better to pay $4 for 15 hours of entertainment or $60 for 20 hours). There is also a "Whored Mode" which is similar to horde modes from shooter games, but I never got around to that or the co-op mode because my friend has the game and I just wanted to beat it while I had it.
Anyways, play this game because it is highly entertaining and not every game should be completely serious. After all, they're just games. (Not to say I don't respect great storytelling games, this game is just awesome).