I beat RDR over the weekend and after close to 35 hours of playing it, there is still so much left I could do in the game. I finished with only 89.1% complete, and the last ~11% is pretty vast. I'd like to try to get it done, but for now I need to move on to a different game (Alan Wake) and play RDR in bursts.
I still have a bunch of achievements left, mostly online ones...I don't think I'll ever get most of those since the online is funky right now, but I'd like to.
If you were a fan of GTA4, you will either love or hate this game. I liked GTA, but felt it was too "busy" and just grew kind of annoyed with it. RDR to me felt more like Bully than GTA, and that's a good thing. However, riding around on horseback isn't for everyone. You'll know in 30 minutes or so if you're going to want to put in for the long haul on this game. If you don't like it, it's not going to "get better" or anything, the game starts you off with a great sense of the rest of the game.
As for the ending, I won't go into it here, but I loved it. Lots of folks did not, I thought it was a brilliant move.
My XBL list is full, but if you're one of the lucky 100 and have the game, let me know and we can play online sometime to try to get some of those peskier achievements...