In all honesty, I was dreading reading and reviewing this book thanks solely to the AU attached to the title. However, Al Ewing and Butch Guice pull off an impressive feat in "Avengers Assemble" #15AU by delivering a good comic that ties into a major event, but more importantly one that can also be read on its own with satisfying results.
Ewing does some some smart things that make this issue of "Avengers Assemble" (which should more accurately be titled something like Captain Marvel and various magical English Heroes Assemble) really work in a way that few comics tied to a major event can manage. Captain Marvel is displaced in England when the Age of Ultron event takes place, and she gets stuck there. Since the Avengers don't come running when things in England go to hell in a hand basket, Carol must assume the worst -- i.e. that the US is in even worse shape. Therefore it's up to her and Captain Britain, as well as Excalibur, Black Knight, and a teenaged girl named Mel to save England from complete takeover.
Ewing's best move, other than writing great character dialogue that finds some levity despite the grim circumstances, is in introducing Computer Graham, a former "hero" of sorts. He's a great POV character for the reader, as he's new to this conflict and so he has to be given the very quick scoop on what the stakes are, which explains the book cleverly without needing pages and pages of info dumping narration. It feels how it probably would feel if this were actually happening, as opposed to how it often feels in comic books with these kind of storytelling challenges that opt for a trip to exposition hell. So, with the help of Computer Graham (and he's quick to point out that he didn't give himself that name) the heroes are off to save the world, or at least this corner of it. The battle is smart and sad, without completely losing its sense of humor and voice, and perhaps most importantly, it feels appropriately heroic. The end result is an impressive "done in one" that's also a tie-in to a major event -- something I honestly don't recall seeing -- maybe ever?
Guice's artwork is up to the considerable tasks Ewing sets for him. The character work is strong, the action scenes make sense, and the storytelling flows well, despite so much ground they have to cover in so few pages. In the hands of a lesser artist, this could have been a nightmare -- it's a lot of plot and a lot of "new/unknown" characters for typical Avengers readers -- and Guice handles it all with seeming ease.
All that said, "Avengers Assemble" #15AU is not a comic book that will blow you away. There's nothing here that will leave its mark on you forever, you may even forget about it not long after reading. But it is a good solid comic book, that manages to tell a nice heroic story, and excels at telling a complicated one that any reader can pick up regardless of irritating event tie-ins.