So far,
Red Undead Redemption, er,
Undead Nightmare has been really good. I've only played half an hour or so, but I like the atmosphere they've got going. It takes itself less seriously than the main storyline, and has more of the "American jackass" humor I loved so much in the GTA III era. The additional survivors in Blackwater were pretty hilarious. Things always go from seemingly normal to a sense of wonderment how these idiots have lasted as long as they have. The writers kept John Marston's dry wit. I laughed out loud a few times; I loved, "Jack, be kind to your mother. Abigail, teach your son right from wrong. ...Quit biting chunks out of other people." The horror movie logic is also welcome: "No, no. I'm just going to go fetch my bag on my own,
down this apparently deserted street..." 
The game's harder than the vanilla storyline's, though! I guess you're mainly s'posed to RUN AWAY where zombies are concerned. Ammo is freakin' scarce. The couple of times I've stayed too long to loot corpses, I've been overrun. Even out in Tall Trees, endless bursts of zombie bears and cougars are showing up, and even where I was supposed to be hunting a Sasquatch, there were a half dozen human zombies, which were replaced by 10 and then 15 or so, and then more... I tried to flee on foot, but a couple Bolters got me.
The design divergences are also interesting. There's no rudimentary camp and I've not seen a shop so far. Since John can only Save at a safehouse, and those are only available at "safe towns." He can only fast-travel between these safe towns as well, and apparently they don't stay safe indefinitely, because there's an Achievement for having all the towns safe at once ("Spinning Plates"). So there's more risk by not being able to frequently save, less ammunition and no means to purchase it, ammunition becomes a currency since turning a town to a "safe town" requires relinquishing your own ammunition or finding a cache.
