For a DLC pack that was announced even before the original game was live, it's quite relieving that Zombie Island of Dr. Ned isn't just asset recycling. In a whopping 1GB package, Gearbox delivers a great deal of new artwork, funny writing and really excellent voicework, so at least in that regard, Zombie Island doesn't disappoint. The world and quests are once more filled with silly puns and pop-culture references, which continues the base game's tradition pretty well.
Everything else also does that, and that's where it starts to reek. Ultimately, Borderlands is a pretty shallow experience - even if CliffyB seems to think it's "Diablo for the FPS generation", it's just not much more than a FPS with some very mild Diablo influences, and obviously Zombie Island isn't here to change that.
First of all, the zombies thing. There are different types of shambling undead: the basic zombie just walks slowly towards the player, inflicting quite some damage once it reaches melee range. There are fanatic zombies that work exactly like the original Fanatics crossed with zombies, and there are Mini Zombies that are a mix of these face hugger scythids that jump at you and Mini Fanatics. Some zombies also spit at you, slowing your movement and clouding your sight. There's more, but these are the monsters you usually fight here. Since they don't shoot, the developers just throw a ton at you at once to offset that you don't need to take cover. Most fights consist of 10 or more enemies that do nothing else than slowly walk towards the player to either melee or spit on him.
This could be fun if the content would scale in level like you'd expect it to. Sadly, this isn't the case unless you finished the main story twice. You can reach level 50 way before you get to the final boss the second time, so you'd be still on Playthrough 2, and that means all Zombie Island monsters are between level 42 and 45. Of course, a level 50 character has absolutely no problems killing most of the monsters here in one or two shots, and considering how easy the original bosses were even on proper level, expect some more who are even less a challenge. With low enemy levels, the loot they drop is also low level, so there'll be nothing of value to you. Since Borderlands builds heavily on the player wanting better and better loot, the whole concept falls apart here. You could argue that Gearbox expects players to finish the game twice to properly experience the DLC, but that's really a bit silly.
What's also a bit strange is the inclusion of a quest series that asks the player to gather zombie brains. These drop every time you dispatch a zombie via headshot, which would be okay, if it wasn't for the fact that you need alot of these brains. Using your special ability doesn't usually reward you with brains, because except for the sentry gun, there'll be no headshots, so you'll need to shoot the zombies manually. And if you happen to stumble over this well hidden mission after you did all the other content, you'll be in for a surprise: while you can gather zombie brains even without having the quest, they don't count.
Of course, the content could be rather challenging at appropiate level, which makes it even less understandable why it wasn't possible to just scale the levels to start at, say, player's level minus three. For gamers still leveling their character, the addon is likely a fun alternative to the already known path, but for level 50 characters, there's just nothing rewarding in there. Oh, except for a new Claptrap rescue that actually awards another three backpack slots.
Lastly, there's no New-U station around any of the zones besides the one you get ported to in order to start the story. It almost seems deliberate, because that way, the content lasts a bit longer due to all that walking. Since there are no cars here either, this gets annoying rather quickly. At least the zones aren't that huge, but when you have to run several times through a flock of insanely annoying Corpseeater birds just to reach a zone entrance, you'll really wish for a teleporter.
So in conclusion, I'd say that if you have a character below or at level 42, or a level 50 on Playthrough 3, the 800 points are very much worth it. For everyone else, it's a mixed bag: while the story is nice, it's also over very soon, and you'll leave empty handed. The achievements or trophys unlock along the story, so there's no additional replay value.
