First : Intense pleasure at meeting all the familiar characters again!
Turns out everyone has teenage kids now, Harry sucks at parenting and - alas! - his scar still hurts from time to time.
Then : Sucked into the adventure, head over heels!
The story is fast-paced and takes you for a dazzling ride down that old memory lane, revisiting key moments from the books.
Why it works : Because the Harry Potter books have always been about three things; characters, story and dialogue.
Rowlings has never been Real Literature. Her writing style has never been "charged with meaning to the utmost possible degree" (quote Ezra Pound).
Removing it, therefore, is no major loss. As the director and playwright seem top-notch, this probably rocks the hell out of everything else in West End, right now.