It's the apocalypse. Bring a mop.
It has to be said that Undead Nightmare is incredibly atmospheric. The opening cinematic creates a sense of impending doom that few big-budget movies can top. Soon after, players are treated to a graphic experience that succeeds where so many other games, movies and novels have failed: We are reminded that the true horror of zombies does not lie in them being ambulatory, cannibalistic corpses, but in the tragedy of a good, beloved individual being reduced to a lower-than-animalistic state of thoughtless savagery and sentenced to an interminable existence of absolute conformity, simply because they had the misfortune of being attacked by a psychotic freak. Undead Nightmare is almost worth the price for this alone.
The meat of the game is concerned with "saving" areas where bands of survivors are holding out. These are invariably towns, cities, and gang hideouts. Most of these areas will be "under attack" when you first encounter them, and you save them by killing zombies until you have filled the "town safety" meter at the top of the screen. (The option also exists to supply the survivors with ammunition, but I have not discerned a benefit to this approach. They give you support fire regardless, and finding your way up to the rooftops to talk to the survivors is time-consuming and aggravating.)
"Saving" is a misleading word, however. An area that you have saved can come "under attack" again, and if you take too long to respond, its status will change to "overrun", which means it will take longer for you to fill the "town safety" meter when you go back. ...While this sounds annoying, it never truly becomes so. Area status changes happen approximately every two days (in-game time): Once every two days, a "safe" area will come "under attack", and you will have another two days to respond before it becomes "overrun". The days are so long that you can probably complete half of the Undead Nightmare campaign within the two day period, so you are practically free to repel the encroaching hordes of undead at your own leisure.
Moreover, saving areas is more-or-less optional. Saving an area will grant you a new weapon, a safe place to sleep (to save your game), and access to ammunition chests that sporadically replenish themselves. When you die, you have the option to respawn nearby with virtually all of the progress that you have made since your last save, so unless you are actually turning off the game, finding a bed isn't critical. Your only real motivations for saving an area are weapons, the "Spinning Plates" Achievement, and a hero complex.
Combat is unpleasant at first. Controls that were 'quirky' for wild west shoot-outs are downright inappropriate for crowd control. Exacerbating the problem is the fact that putting down a zombie requires a headshot. Putting enough lead into its ribcage will do the job eventually, but when you finally turn your smoking steel on the rest of the horde, the last thing you hear will be 'click'.
A big deal has been made of the shortage of ammunition. Well...the dead are roaming the earth...bullets SHOULD be hard to find. That being said, while ammunition does not come as fast or as freely as in the original single-player game, it is never problematic provided you don't start blasting indiscriminately at the putrid mobs. A cool head, a steady hand, and lots of guts will prevail here. The Dead Eye meter is extended in Undead Nightmare - make use of it to thin out the larger groups before getting in close and picking off stragglers with the close range insta-kill. It is a repetative tactic, but it is also exciting, and a good bit of skilful maneuvering is still required to keep from getting completely swarmed.
A pleasant benefit of the abrupt, violent and total collapse of social order is that the people charged with protecting the law are too busy protecting their skins. You can murder anyone - in public, in broad daylight, in cold blood - and never fear the long arm of the law. The downside to this is that the thrill of being bad is gone. In fact, the morality system is gone completely, which is a shame, because it would have had so much potential for determining the ending. ...Outlaws are also rather preoccupied, so, except for a few random encounters, you never have to worry about being shot at deliberately (survivors don't really care if you're standing between them and the zombies).
Much like in the original game, wildlife will prove to be a hazard should you get off your horse. Packs of undead coyotes, wolves, and even the occasional undead charging bull will spawn into to give you trouble. But the difficulty is greatly increased because the undead animals require headshots as well. At a certain point, it just isn't worth stopping between your appointments.
Fortunately, stopping is optional as well. It wouldn't be the apocalypse without the Four Horsemen of.... The Horsemen themselves don't make an appearance, but their mounts do, and if you can find and break them, you can ride them. Each horse has a special power, the most useful by far being War's ability to set enemies on fire. But once you break a new one, your old one disappears, and there is no option to go back. You just have to enjoy the burn while it lasts.
(Spoiler section begins.)
Rockstar have always had a...different approach to story-telling. Theirs is an approach designed to propel the player around a large world over a long period of time. When that period of time is drastically reduced without a commensurate reduction in world, the results are less than encouraging. Yes, you will have free reign of the entire map. Just about all of the surviving characters from the original game make an appearance, if only so you have a chance to kill the bastards you wanted to kill the first time.
Undead Nightmare is essentially an alternate ending - it starts after John Marston gets his family back and before the American Army shows up to kill him. When his family gets zombified, he sets out to find a cure. In nearby Blackwater, a group of survivors blame the undead plague on a "cursed glass eye," a "snake-oil salesman," and the Mexicans in turn. When the story is laid out for you right at the beginning like this, you are justified in having low expectations.
You head south into New Austin and kill time until everyone agrees that you should go to Mexico, at which point you cross the border with a group of people who are scared to death that the zombies will somehow catch up to, board, and stop your train. Yes...zombies versus train...it's pretty funny, but I don't think it was supposed to be.
Once in Mexico, a new character is introduced with no subtlety whatsoever, and she makes you aware that the former rebel leader and new president of Mexico, Reyes, brought about the undead plague when he opened a cursed Aztec tomb, etcetera. Here we have a golden opportunity to delve into some tasty, Aztec mythology-driven narrative, but instead we get a stroll through some underground tunnels with the new character in tow as she endlessly spouts the same three platitudes. The Aztec tomb could have even made for a good Indiana Jones homage (which I think is what Rockstar intended to do), but as it is, it's just dull.
I don't think that the problem was laziness - I think it was restraint. Rockstar are better known for gritty (pseudo-)realism - the realm of mysticism is new to them. They didn't know how to approach it, so they threw a little bit of everything at it: American folklore, Christian mythology, Aztec curses. What they ended up with is not only inconsistent (how exactly does an Aztec curse conjure creatures from Christian mythology?), but never really develops into anything deeper.
And speaking of lack of development, it is extremely difficult to view John Marston as a sympathetic character when he repeatedly stands back and lets innocent people get masticated. I'm not even joking. It happens in every other cinematic, and it is quite infuriating by the end.
(Spoiler section ends.)
The above tirade concerning the story aside, Undead Nightmare is a great buy. You get a lot of stuff to do, and a chance to tie up some loose ends. If you like Red Dead Redemption and zombies, you will probably think that Undead Nightmare is the greatest thing that has ever existed. Definitely recommended for anyone who enjoyed the original game and who wants to go back for dessert.