Simply put: Grand Theft Auto set in the wild west. Thus, if you like the GTA genre, keep reading.
The best part of this game is the storyline. It's very good, and really well paced. Despite a slow start you're left with no questions at the end. Granted, the beginning and the...sort of end...(no spoilers) kind of ignore the gameplay mechanics, but it works. The story also sets up both a prequel and sequel.
The gameplay is exactly what you'd expect from the genre only the vehicles are horses and the weapons are from that era.
They do a great job with their version of bullet time and it will be necessary to use it during your time playing.
There are a LOT of missions within the story and tons of stuff to do on the side.
Not sure what else to say about it, but it's a great game.
The only negatives are spawning enemies, particularly bears when you're out hunting. You can kill a few and out of nowhere one will teleport in behind you. That can be annoying. There are probably a few more that I'm not remembering at the moment, but they're minor.
An extremely well polished game with an excellent storyline.
9 out of 10.