The Trip Advisor reviews might have set expectations too high, but I didn't think the tour was excellent. It was okay. Maybe it's because I'm not a big fan of Harry Potter -- I've seen all the movies and enjoyed them, but I'm not the type who would would memorize book/movie trivia, immediately recognize scenes, or want to sort out if the guy beside me is a Hufflepuff or not. For moderates like me, this review is for you.
I thought it would be an interesting way to see London, and we did get to see a number of places that should be interesting for a London first timer. The thing is, the historical or cultural significance of the places that are stops in this tour are not really the focus of the tour, so it will feel lacking to someone who is interested in history or culture apart from the Harry Potter aspect. Tip #1 is go on a proper London tour; this is not (as it is not probably meant to be) a substitute.
Tip #2 is to bring an umbrella just in case it rains, maybe some water and snacks. The tour does take a while.
One large source of disappointment is that the scenes that were filmed in London, and were points of interest in this tour, are not always the memorable ones in the movies. Thus, if you are not really steeped in the Potter series, you might feel that of about 10 stops, only a couple will resonate. Most of the places were made to be a bigger deal than they were. I'm not sure if it's in the presentation (e.g., instead of starting off with, "do you recall the scene in this movie, where [blank] did [blank]? Well it was filmed here." what you actually get is, "Do you recognize this place? [We look around] it's that scene from this movie."). If you don't remember the scene, the presentation doesn't help you remember it. Expectation builds up with the mystery, and if it can't be recalled when the information is revealed, it's a letdown at times. The reaction might be, oh, okay. And when people can't remember (in our case, there were about 4 kids and a lot of adults), it felt like our lack of recognition dampened our guide's enthusiasm, too.
Tip #3 is for the tour provider: A lot of people have watched these movies and appreciated them, so they will join your tour. Unfortunately, not all of us know what we're looking at, so it might help if our varying levels of expertise in all things Potter can be considered/not taken for granted.
I did like how the guide was really reaching out to this sweet German-speaking girl in our tour, and how she tried to engage us even if at times we looked blank. It's a tough job, and our guide did hold up until the end.
If someone asks me if I would recommend this tour, I would say, it probably only works for people who really read the books and watched the movies several times. And a trip to London can still be complete if you bow out of this one.