I saw it on the very first day of previews and absolutely loved it!
Sure, when I got out of the theatre and thought about the plot for a few days I had massive issues with inconsistencies between the show and the books, and plot holes I couldn't wrap my head around, but in the theatre it was actual magic.
Most of that was due to the fact that it felt like real magic was happening. Stunning, stunning show, technically. Most stage tricks I can figure out roughly how they did it; most of the Cursed Child bits of magic I couldn't for the life of me unravel. I've put an example under the spoiler cut (no actual spoilers, but I want to be careful not to ruin anything.
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There's a moment where Hermione gets sucked into a book case, through a gap the width of one book, and honest to god it looked like she went through it. I have absolutely no idea how they did that.
I'm pretty alone in my appreciation for Harry's characterization, but I thought the writers did a great job of addressing Harry's lifetime of trauma, and the oft-manipulative relationship he had with Dumbledore. Some other characters I'm not so keen on, particularly Ron. Albus and Scorpius were very sweet (largely because of the actors' chemistry and stage presence, I'd say) and I enjoyed their inclusion significantly more than I thought I would. There's actually about an even balance between old characters and new, which surprised me, but I can't say I'm complaining.
At least when I saw it, there was a lot of unnecessary dead space with a bunch of staircases rolling around the stage every few scenes, which made the action drag and honestly didn't feel necessary. The first show does have more tricks than the second, and a more well thought out plot, so I definitely enjoyed Part 1 more than Part 2.
Overall, the entire show is beautiful, and it's entertaining if nothing else. If you haven't already, I would recommend not reading the script if you're planning on seeing it. The plot isn't something you really want to scrutinize too closely.