I went to see "Book of Mormon" last Friday and thought it was perfect.
What I took away from it was a clever satire on the horrible conditions in Uganda, and a nice message about the Mormon religion. After the play, I discussed the play with those that came to the show with me.
They expressed that they felt the happy ending was done out of necessity to avoid upsetting anyone as 'bashing' the entire time would not have been acceptable. I feel this opinion misses the powerful message of the play: The religion is based on broken logic, but the teachings are quite applicable and to everyone's benefit.
First to address the statement that they wouldn't just bash a religion without a happy ending. Matt and Trey have a long running show called "South Park". There was one episode where they addressed scientology and bashed it for being BS (and suggested that Tom Cruise was gay) pretty much the entire time. At the end, Stan (a main character in the show) openly stated in front of the followers that the religion was one big scam. This episode should disuade anyone from considering that they (Matt and Trey) would avoid purely bashing anyone (that didn't deserve it).
There is another episoide of South Park that further exemplifies what I took away from the play and that is actually when they deal with the Mormon religion. A new family comes to town and the new kid is very very polite to everyone. He is originally not welcome in the group and Stan is sent to beat him up. The kid is very polite and even invites him to dinner. The show goes on and Stan's family looks into being Mormon because of how happy this new family is. At the end, Stan can not handle the broken logic behind the origin of the Book of Mormon and denounces the religion, claiming the family is tricking people into their way of thinking. The new kid finds Stan the next day and explains that while the logic may be broken, its teachings are what matters and what keeps everyone happy.
The Book of Arnold exemplifies this point very well in that the story is not what matters, but the teachings (which in the case of Uganda, were very specific to their conditions and spoke to them in an effective way).