My main issue with this film is that I absolutely loved 28 Weeks Later and I thought it served to add a LOT of interesting lore to the universe. (Plus, it helped create a sort of "formula" for the 28 series.) And 28 Years Later effectively says, "Nope, that movie never happened. Forget about it." Which, I can understand why the director chose to do that since he didn't work on the previous film, but for fans of the series it amounts to a cardinal sin to overwrite a film just because it had a different director.
The first hour or so of 28 Years Later has so much potential and it laid the foundation for a really powerful story. The problem is: they went into this project knowing that there were going to be 2 more films after this one. And that is a fatal mistake for any filmmaker. When you're certain that your film will get a sequel, then you don't have to worry about making one cohesive story with a beginning, middle, and end. You can just make a beginning and middle, and then end the film at (or near) it's climax. I like to call this the "Rebel Moon Syndrome".
Thematically and stylistically, this film is excellent. It feels like it belongs in the series with it's jerky, intense cinematography and it's English punk soundtrack. But that is all it really has going for it.
It's far too short to feel satisfying. The pacing is just downright awful and scenes are only barely connected to one another. And all of the film's metaphors and it's motifs are either spoon fed to you or outright told to you by a character.
Overall, I don't regret seeing this film because it made me appreciate the previous films more, but it certainly wasn't worth paying movie theater prices to see it. It was easily one of the most "mid" films I've seen this century. I can't recommend.