I have sat through a lot of theatre over the past 40 years or so, and I loved Mamma Mia! My good friend convinced me to see it, because she told me it was the show that got her out of her 9/11 slump. I went in Minneapolis and thoroughly loved it.
I saw it again in London the following summer, and once again, loved it. I've gone to see it twice in New York, and I plan on seeing it again in Minneapolis in June, because I'm going with people who haven't seen it.
It's not a great plot, but the production is so much fun. I was never an Abba fan and I loved the score. The woman who sat next to me in London said she'd seen the Queen and Rod Stewart jukebox musicals, and in those shows, the plots were forced out of the songs. Not so with Mamma Mia.
At the end, the cast has you up and dancing and you leave the theatre in such a great mood! Fiddler on the Roof, Cabaret and Les Miz don't do that!
Go see it. I think you'll be as pleasantly surprised as I was and have a really great time in the theatre!
NYC Visitor and Broadway Fan