If I would have to sum up this game in just one sentence I would say it's GTA with horses instead of cars, but that wouldn't do the game justice. If you've ever played any of the recent GTA games, the gameplay will seem familiar to you. The thing that really makes this game stand out is the atmosphere; you really feel like you went back in time. The towns are rich and detailed and the landscape vast and beautiful, if not a little empty at times. Missions are interesting and varied, but if you get sick of doing missions the excellent little mini-games, like poker, are fun as well, not to mention the various side-missions and the multiplayer mode.
The graphics are great and the characters are detailed. The voice acting is very well done and fits the setting perfectly. If you like sandbox style games, definitely pick this up. Even if sandbox style games don't strike your fancy, consider giving it a shot, as the unique setting definitely makes it stand out from games in this category.