Aside from Escape from Butcher Bay, this is arguably one of the most immersive games ever.
The SP campaign is not the easiest game and not polished around the edges (a couple bugs, and the interface can be clunky in spots) but considering the game almost never saw the light of day, it still has a sense of cohesion and fun. You will want to jump right back into the game again as soon as you finish it.
The AI is very good, and that's a big plus as you hire muscle to assist you. Learn to use them wisely - they never run out of ammo and you can heal them. I doubt anyone could complete the game without them, but you are welcome to try - which leads us to the next paragraph of this review.
Re-playability is high due to a couple open-ended tasks, level difficulty select, weapon mods, and an often negligible NPC interaction system. A couple times I found myself at the cut scene (a couple of which are very cool) ending a level, yet I still had not explored everything I wanted to.
So I rate this game a 9.0 as I believe it manages to be mostly non-repetitive, yet allows the inner thug in you to bash a few heads and cap a few goons in your quest for revenge and working your way to the top in Poisonville. It's one of my favorites, no doubt.