Given the fact that the script sold 2 million copies in 2 days, I would call that significant buzz on its own. At any rate, I saw both parts last Saturday and here are my thoughts (without spoilers)...
This really doesn't have the feel of a "play" in the conventional academic/critical sense. If only viewed from that perspective, the script is definitely weak. Much weaker than the books and films that precede it. But what is clearly evident, is that the text was constructed as a love-letter to Harry Potter fans. To me, it was obvious Rowling came up with a very basic framework for the plot and characters while Thorne was tasked with referencing all 7 books within the plot, linking them all together, providing closure for beloved characters, and shoehorning every memorable magical device as possible in order to provide fans with a live Harry Potter experience that feels old and new and complete and fulfilling in order to achieve the impossible...to please everyone. And it mostly succeeds.
It really doesn't take long to figure out the path on which the plot is laid and if you're even a little bit familiar with the stories, 90% of the script is quite predictable. A couple of the new characters are actually very strong and memorable, given the attention and care we expect from the novels, while others feel like afterthoughts or "devices" that seem either out of place within the established world (either in text or in visual design) or underused and erroneous. One in particular (and this is no spoiler) appears repeatedly to represent the passage of time or something and it's never really clear what or who he is.
That being said, nothing can change the fact that viewing the entire play is genuinely thrilling and immensely entertaining. The pointed references to familiar spells, characters and scenes plays on the nostalgia of fans and while they could have come off as intrusive pandering, they are so cleverly staged, each is delivered like a beautifully wrapped gift to the audience. And from the very first instance, the audience enthusiastically shows its gratitude. By the end of both parts, I have truly never heard a West End audience respond as enthusiastically to a show, play or musical (or rarely a Broadway audience, for that matter). There are a couple of slow and static spots, but those could be easily remedied in staging and direction alone. Everything else moves at a brisk pace with scene changes choreographed suspiciously similar to Curious Incident. The cast is mostly perfect with only a couple of people not quite hitting the heights of the rest. The actors playing the might trio (Harry, Ron, Hermione) and the two boys (Scorpius and Albus) rise to the top and I would expect some nominations for at least two of them. Oh, and I actually hope they release a CD of the music, composed by Imogene Heap.
The visual effects, whether they involve the cast or not, are at turns breathtaking, clever, funny and truly wondrous. In many ways, this show plays as an "experience" much like Lord of the Rings (which, to my own surprise, I loved) did in the West End almost 10 years ago, rather than fit comfortably in any play/musical category. Though clearly, Harry Potter is already far more successful.
Personally, for Harry Potter fans, you definitely get your money's worth and more. For those unfamiliar with Harry Potter, you'll be confused, but still entertained. And if you are an addict to the point that you need a roomful of entirely new Harry Potter merchandise, you won't be disappointed by the merchandise. Just note that the souvenir brochure is ONLY sold after the end of Part 2, so be prepared to wait in a long queue if you want one.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian