I've played it, and to me it trumps the GTA games. It is basically the game that came to be with all the lessons learned from the GTA series, only they set it in the Old West (which was starting to fall apart at that point).
There are large open stretches of land but I must say I never really got bored. There are always animals to hunt, bandits to kill and people to help. At one point you know exactly what happens in certain situations but otherwise it is fun. They also put up some challenges (kill x amount of this creature, kill a creature that is really dangerous with just your knife, shoot the hats of people) so you can keep busy with that.
The story itself is very well done, although you kind of know what will happen in the end (although this still has a twist to it), and you will start to like the rough and tough Marston. They do a good job of explaining his history and how he wants to be done with all the violence but gets pulled back in all the time, also of him as one of the last cowboys/outlaws in a world that is rapidly changing and doesn't want nor need men like him anymore.
The humour in the game is also very well done (especially the movies they play are very fun and the conversations in between) and can also be found in the achievements/trophies. (You can get one if you hogtie a woman, take her to the traintracks, place her on it and watch a train ride over her).
There are also a lot of minigames you can play although a few take some getting used to, there are some gun duels here and there and you can clear out nests of bandits.
I really enjoyed the game, thought the ending was well done and they took a risk, although I was less happy after that was done, but that was also in part because I spent hours upon hours just riding around and hunting and all that. I spent so much time in the game that I was pretty much done with it.
The multiplayer is okay, if not a tad hard to get into. You can either do free roaming and just roam around and do your thing and clear out bandits nests, or join a posse (or create your own) or get into one of the multiplayer modes. Most of them are variations of regular multiplayer matches (like collection gold and having to bring that to a collection point before they shoot the hell out of you). Thing is that you start out as a greenhorn with a mule and a simple gun and collect more and more better horses and guns as you progress, but many others you will be playing against will have the top of the line weapons and will shoot you square in the head from halfway across the map.
All in all I find it to be one of the best Rockstar games around, mostly because they really looked at what worked and what didn't in the GTA games and made it better in this one (gives good hopes for GTA V), has a good story, characters and things to do outside of the story and it gives hours of entertainment.