Way back before the days of Grand Theft Auto V, Rockstar understood what DLC could do. Instead of the usual, horse armor or one extra level or some such; Rockstar instead opted to expand it's stories by adding more content or simply make a new scenario with that engine. Thus Undead Nightmare was born, and it was so good and different that it just warranted a separate release, and was my introduction to the series.
Red Dead Redemption, by and large has a lot of content, and in comparison to it's sister game, Undead Nightmare doesn't. While I don't think Undead Nightmare is bad about what it wants to do, the comparison of side missions, story, and characters simply feels more bare. Undead Nightmare has such an interesting take to it's world, and the fact that zombies and western theming is rare to began with is kinda sad. Being able to see old timey folks react to a zombie apocalypse, and try to explain it with what little they do know just makes for some interesting dialogue and test of their character. Undead Nightmare knows it's a unique scenario, and rides that fact each chance it gets with each new mission. The sad truth is though, it just isn't as long or as meaty as you'd want it because it still feels like there is more to do with this scenario than the game let's on.
Undead Nightmare being built on the same engine of Red Dead Redemption means that the gameplay is similar in every way. From shooting, riding horses, walking, talking, catching, and hunting; Undead Nightmare plays like a GTA game in the west, but this time there are also supernatural creatures about. Perhaps the only sidequests really when it comes to this DLC, The fact that their is so many new horses and creatures based off mythos, and that each creature has quite the aura about them makes for such an interesting sight when you finally see one. Add that together with the dark and gloomy atmosphere Undead Nightmare as makes for such a wonderfully subtle tone shift that is just kinda impressive to see when you compare the two games.
In the end, Undead Nightmare does what it set out to do, provide an interesting scenario for the player to explore if they already beat Red Dead Redemption. While you can play Undead Nightmare without any knowledge of the main campaign, there is just a certain interest seeing areas and townsfolk react to the infection that comes with that knowledge. While Undead Nightmare ends a bit faster than I wanted it to, it still provided an unique experience, and really that's all that matters.

Entertaining, but awfully repetitive and frustratingly buggy. Almost every mission, main and side, boils down to "shoot zombies," and when the piece of shit glitches out and makes it so killing zombies doesn't contribute to the progression meter, it quickly gets tiresome. At first I thought it was just the totally legitimate copy I was playing on my totally stock, totally not modded 360, but sadly a quick Google search confirmed that it was indeed just how the game is. This made the relatively simple task of attaining 100% completion a Herculean effort, and I really don't recommend doing so.
Along with the repetition come some other lazy design decisions, like the fact that, out of four types of undead, two have the unique characteristic of charging at you.
As a whole I don't think this game's entirely worthless, as I've said it's rather entertaining with a fun story and dialogue. I just wish the wrinkles had been ironed out a little, and maybe a little more variation were added.

Oh, and Blood Dragon is still the king of expandalones.

Undead Nightmare has a fantastic narrative that feels like something straight out of a B-Movie thanks to a hilarious cast of characters. However, it is let down by repetitive mission design and too much time spent riding from point A to point B. There are some really interesting enemy designs and gameplay elements, but I just wish these were used in more interesting ways. What’s more I had several glitches, such as infinitely spawning headless undead preventing me from completing checkpoints without reloading my save.

This has all the pieces to reach the highs of the base game, but unfortunately it fell short for me. There are some great moments that are worth experiencing, but some weaker elements and the prevalent glitches really hampered my experience.

Why have I never played this before???

'You eat babies!'
'I'm going to wander down that lonely, deserted street and get my bag.'
'I don't like Jews. Or colored folk. Or natives, now that you mention it... I bet you like Catholics. Can't stand them neither. Nor women, Fabians, Socialists, homosexuals, Asians, or British.'

Absolute banger, but I should've played during halloween for better atmosphere. Wishing for Undead Nightmare 2 but it's never going to happen. Just like the GTA V DLC we were promised. Now we just have to pray that Rockstar find it in their hearts to give us disgusting masses some GTA VI Story DLC.

Played on Xenia Canary emulator for PC

It's a very creative DLC in concept, but kind of lazy in execution. The whole game is basically going around saving towns, which is always the same. Even the main missions feel like the most basic Rockstar side-quest possible: "go do this and bring me some of that", and it gets a bit boring pretty quickly, especially if you play this right after completing RDR1.
Still, as I said, the idea is very creative. Mixing western with zombies and some ancient mexican curse is cool af. The humor is on point as well.