Jason The Commenter (@BXGD) mentioned, when he saw The World's End, that it's best to go in not knowing anything about it, and there's some truth to that. So if you like going in blind, you might want to stop reading after the next sentence, which is: My caveat to that is that if you're familiar with the Simon Pegg/Nick Frost/Edgar Wright oeuvre (Shaun of the Dead, Hot Fuzz and "Spaced!"), you can't be too surprised by what happens.
Nonetheless, the movie has a bravura first act ending which hooks you into the rest of the film. Then, as @JulesLaLaLand noted, it kind of falls apart at the end of the third act. (Shaun and Fuzz suffer similarly, though not nearly as badly, from this Need For The Big Finale syndrome.)
Anyway, the premise of "The World's End" is that loser Simon Pegg gets his old teenage gang together for one night of epic pub crawling called "The Drunk Mile". Wait, no, "Golden Mile." (I think "Drunk Mile" would be both more honest and less gross.)
Of course, even as he wears the same clothes, hairstyle (sorta, he's bald-y now), and drives the same car, his friends have moved on and had real lives. So the comedy aspect is tempered with a kind of poignancy of a life wasted.
But that all changes soon enough and the movie goes off in a completely different and amusing direction that keeps the second act popping.
I can't really say much about it without spoiling it, but if "Shaun of the Dead" recalls classic Romero zombie movies and "Hot Fuzz" is basically a fun update of "The Wicker Man", this movie strongly recalls—well, again I can't say, or it'd spoil it.
But you'll see it, if you look.
We enjoyed it but I was inclined to think it was the weakest of the so-called "trilogy" (Caveat: No actual narrative connection to Shaun or Fuzz)—but it's the sort of movie I'll watch again to see how I feel later on. I think I felt slightly let down by Fuzz after Shaun, but on multiple viewings I think it's the strongest of the three. (Definitely best, and lowest-key, ending of the three.)
What I particularly enjoy is how the actors change characters from movie-to-movie. Nick was a gross loser in Shaun and a childlike naif in Fuzz and a no-nonsense businessman (though still somewhat idolizing Pegg) in World. And he doesn't even consider himself a real actor. (How English!)
Similarly, Pegg plays a low-ambition retail manager in Shaun, a super-cop in Fuzz and a burnout in World. And basically I think they didn't use much makeup, so he looks all of his 43 years and then some.
Even Edgar Wright, whose signature cuts and camera moves powered the hilarity in "Spaced!" and Shaun hearkens to these techniques here without leaning on them. These are not one-trick-ponies. They have a style but they're not limited by it.
Bill Nighy has a fun voice-over only role.
Thing is, if you like these guys, of course you're going to want to see it. And if you don't, you won't. But if you don't know, it's really hard to say if you will. There's really nothing quite like the stuff these guys put together.