Saw this last night and it was a very fun, but not at all deep, musical. The show almost reminds me of the vibe people describe for On Your Feet! in that there were many moments when it felt like a concert, in a good way. The songs are overall very fun, though mainly because of the extraordinary children. If Only You Would Listen works very well as a serious song in the first act. Whatever song it is the principal sang in the second act did not.
As many have said, Sierra Boggess is woefully miscast as Miss Mullins, and as a result the role suffers. You can tell Webber adjusted the role to fit her, and the changes were not for the better.
Part of the problem is the show tries to throw in some serious moments, but doesn't earn almost any of them. None of the children really have their parental drama fully develop, which would be much more intriguing than watching band practice 18 times. Miss Mullins is also not in the show enough to develop that (to quote the show) "bitchy" vibe, or to build any chemistry with the lead.
The show doesn't really deal with a strong message either. The main messages appear to be listen to your children, and rock and roll makes the world go round.
All that being said, Brightman does a very good job as the lead, and the kids are the heart of the show. Though I don't think the show was anything particularly moving, I enjoyed myself, mostly because of how talented all the kids were