That was way more fun than I'd expected. The title tells you everything. Apocalypse, megadeath, etc. Marvel's superheroes are zombies. I hadn't, though, expected it to be hilarious.
We don't see them eat the world's human population. They've already finished doing that. This limited series is a sequel to a Mark Millar story arc from Ultimate Fantastic Four 21-23, which introduced an alternate Earth populated by zombies. (It stands up fine on its own, though, and you don't need to read anything else first.) The result is that our hungry heroes are in a world with no easy food sources. What's a poor flesh-eating undead to do?
Also, importantly, these aren't the mindless Romero zombies that Kirkman used in The Walking Dead. Marvel Zombies can talk and think, although they can do it better when they've eaten recently. They have their superpowers. They can deliver heroic battle speeches, albeit with a flesh-eating twist. They're capable of working together to overcome a foe, while being so depraved that they might also kill and eat their own allies. The differences are that:
(a) they're evil and want to eat people. "If we took a Quinjet, it might look like a rescue mission. People would come out of hiding and try to flag us down."
(b) they're extremely hard to kill, even after disgusting comedy damage. (Wolverine vs. the Silver Surfer made me laugh and laugh.) They're capable of researching their own zombie physiologies. They find ingenious, gross solutions to their problems. "You understand, right? I don't enjoy this. I think it's sick. I do it for the good of us all. I like to think that if I didn't keep you so drugged, you'd volunteer for this."
Yes, the gore is over-the-top. That's necessary. That's also why it's funny.
Then, wonderfully, the Silver Surfer announces that Galactus is coming. Yes, the planet-eater. The zombies are themselves about to be eaten. "It is pointless to resist. Your fate is inevitable. I beg you, please. You are lesser beings. You should be honoured to give your lives to provide the world devourer with sustenance."
Theoretically, what follows is a conventional superhero adventure. Heroes fight Galactus! There's a big hero-villain fight, e.g. Spider-Man vs. Venom, or Captain America vs. Red Skull. It's even old-fashioned visually, with Phillips drawing the heroes in whatever classic costumes he preferred, not the latest modern ones. Everything's there in the expected way... except that our "heroes" aren't saving the world. They're attacking Galactus because he looks big and delicious. (There's also a subplot with a handful of uninfected heroes, but that becomes blackly hilarious too when even these survivors briefly start killing each other too.)
This book was a happy surprise... and not just for me. No one at the time expected much from it. Kirkman and Phillips were something like the twentieth writers and artists Marvel approached. People heard the title and had my initial reaction. "Yeah, yeah." It then became the year's biggest selling graphic novel and spawned lots of sequels. DC and IDW have since both done similar storylines... and after reading a few of those (including some of the Marvel Zombies sequels), I've concluded that this original limited series and its black comedy anti-heroes are a bigger subversion than you'd think. Comparing it with its successors has raised my opinion of the original.
But, most importantly, it's funny.