I loved it.. one of the best games I've ever played.
I did not like the zombie idea and haven't played the DLC. It could be fun but I don't think it jives with the 'wild-west simulation' style of the game.
Surprisingly the "Zombie idea" in the "Undead Nightmare" DLC fits pretty well.
Just think about it...it happens in a period where science is still being played around with while also left to the imaginations of uneducated massess of townsfolk... somebody could have been screwing around with something they shouldn't have...
Playing the DLC is like playing another short game, not just an add on. Many of the things you could do in RDR have been altered so that they fit a more apocalyptic scenario that UN brings. One such thing is there are no stores to go to (due to dead shop keepers, I think), so you'll have to find ways to gather provisions. New gameplay, locations, characters and weapons are all added as well.
There's one weapon that does simply outrageous amounts of carnage and is particularly my favorite.... those who have played the DLC would know what I'm talking about...
The experience also brings a feelings of desolation and hopelessness due to the lack of a way to deal with this outbreak in the old West, loneliness and mass paranoia due to the fact that many of the locations you frequented in RDR are being attacked constantly. Major characters and townsfolk are not safe and anyone could be next in turning into the undead. You can be attacked anywhere at anytime....
If you thought the cougar was bad enough... wait until you run into an undead one at night in the middle of being chased by a huge mass of zombies that can run as fast, or faster than you... heh heh heh... Total mayhem...
Give the DLC a try. It may not fit the Wild West Simulation, but I don't think it was created to do so. Don't let the idea of it stop you from playing something that is truly unique... As a heavy player of this game myself I totally recommend it. If you loved RDR, then you'll definitely enjoy UN...
Rockstar made an awesome game... they wouldn't let you down...