I'm no Potter-head, so to speak, by any stretch of the imagination. I've read all the books once, watched all the movies once--and that's about it. As such, I'm often a bit overwhelmed or overmatched by some of the more intense Potter fandom properties. Fortunately, I found this 20th anniversary celebration to be perfectly calibrated to my sort of HP experience!
For a very basic overview, this special is equal parts nostalgic retrospection on clips from the films, and recent interviews with some of the main (most recognizable) cast members. The triumvirate of Daniel Radcliffe, Rupert Grint, & Emma Watson are the most frequent contributors, with Ralph Fiennes, Gary Oldman, Robbie Coltrane, Helena Bonham Carter, basically the entire "Weasley clan", and others popping by to chat about their experiences both on and off set during the making of the films.
Even as a more casual fan of the property, it is clear that Harry Potter books/films have had a tremendous impact on popular culture. In that sense--and along the lines of the Friends TV show reunion that drew up the template for this sort of thing--the actors and participants (and fans!) very much deserved this retrospective. In other words, it has the gravitas to pull it off.
It's obvious right away that Warner Brothers spared no expense when putting this together. The sets are immaculate (mirroring settings from the films) and the whole thing has an extremely high production value, from the clips to the interviews to the music and the voiceovers. First-class all the way.
The hallmarks of this reunion special, though, are the sometimes-emotional, sometimes-hilarious interviews! The leading trio have some wonderful moments together, but then each interact with others from the cast in unique ways. Moments with some of the films' directors are especially well-done in this respect. There were times I found myself belly-laughing; other times trying (and failing) to hold back tears.
The only notable absence here is original author J. K. Rowling, almost certainly due to her political spats of late. Though not present in the flesh (almost certainly a good thing if only for the distraction her presence would have caused), she is mentioned frequently and archived interviews seen numerous times. Rest assured, her presence was not entirely removed or anything like that. A good balance considering the circumstances.
Overall, I was a little worried about being overwhelmed by Potter minutiae from "Return to Hogwarts". But instead I found a delightfully entertaining little trip down memory lane, while at the same time catching up with the present-day actors/directors/filmmakers who made it all possible. Whether you are a novice Potter fan or can win HP trivia contests, I think you'll enjoy this!