How in the hell is the multiplayer fun? It's basically GTAIV's mp and well we know how that turned out.
Different strokes for different folks, I guess. I found RDR's multiplayer (as well as GTAIV's) to be a lot of fun. All I did was the free roam on both game though.
In RDR, it's great when you can get a couple of groups of random players to start fighting (I had a memorable 8v8 at El Presidio, all random players). Sure, there are some griefers (and they are pretty easy to deal with tbh), and the world is somewhat lacking (I wish there were more civilians, instead of just sheriff/mexican army).
I also loved every second of the single-player, and I just recently started going through it again. After the story's over it gets a bit stale, but that is to be expected from this kind of game to be honest, it's not an RPG like Morrowind or Oblivion.