I saw this while it was in previews, and had exceedingly high expectations (As I was expecting a collision of two maximalistic aesthetics [Des's and Wayne's]). The analogy I'll give is I was expecting the second coming of Christ, and all I got was an obscure 14th century saint. As far as reviews, I agree almost entirely with Bob Verini's rather eviscerating review in Variety.
Rather than giving a line by line complaint list, I'll give a couple bullet points...
1: If you go in expecting a remarkably three dimensional Flaming Lips show, you'll likely be disappointed with how much of the show feels trapped within the proscenium arch. So much of the show feels remarkably pulled back and surprisingly understated.
2: My constant line is the narrative is a lifetime movie of the week about a girl with a rare form of lymphoma. Yes there are some explosions of fantasy around that (and that is where the show succeeds the most), but there are nowhere near enough of them, and you're left with swaths of banal scenes where the staging blandly illustrates the lyrics.
I will admit that the last quarter of the show is more successful and emotional (If you know the lyrics, it won't take a rocket scientist to figure out what happens in that part of the show [starting at Feeling Yourself Disintegrate onwards]), but by that point I was well past the point of no return for not liking the show.
Bob Verini's review in Variety
The one show everyone on Broadway is waiting to see: Twyla Tharp presents: Big Bottom - The Spinal Tap Jukebox musical!