I absolutely adore this! I think the part that really gets me is that, even though the story involves two entirely non-magical characters in a Muggle setting, it's still Harry Potter. The look at the magical world's effects from an outside perspective is really brilliant and something I haven't really read before.
I also love the unspoken interplay between Piers and Dudley's friendship and everything deeper that's going on between them. They're absolutely perfect and such teenage boys. It's really a lovely dynamic.