Just downright fun!
Yes, if all you do is stick to the main plot you will beat the game very fast. However, if you work on the side missions as you go (as I did), the game is not all that quick.
Personally, I thought the graphics where great. I think the way they where done gives it a more western feel. (I also really liked the graphics in Red Ded Revolver for that matter) There where a few minor things here and there, but nothing that took away from the game for me.
Riding a horse! That just outright rocked and really made the game and exploring it so enjoyable! Also, doing things like herding cattle was a blast. I thought the overall immersiveness was great.
All of the sound was just great, right down to the sound of your spurs as your rode or walked into that saloon to fight some bad guys. The music was also good but I don't know if it was that prevalent as I don't think I was "paying attention" to it so much. There where some nice touches with some faint Indian music in certain areas, etc.
Overall, I have really enjoyed the game and like RDR, I will be playing it a lot even after beating it, just because it's downright fun to play!