A must have.
The graphics are excellent. The "wild west" atmosphere is excellently done. My only complaint is that there's not enough "world", there are basically two towns and some stuff in between. The sound is also great, and the voice acting is fantastic. That's no surprise, since there are some fairly big name actors doing the voices, Kris Kristofferson and Ron Perlman are a couple examples.
The gameplay is excellent. It's like GTA with horses instead of cars. The game's storyline is great, but a little short. However, this is supplemented with a ton of side missions, such as bounty hunting, being a ranch hand, helping the marshals, and hunting.
I can't wait for a sequel, if there is one.