Mama Mia was a good movie.  The plot was a good simple story that had a lot of heart and character.  The music was good overall, but honestly a little distracting in places.  (especially when Pierce Brosnan was singing.  I just can't compute that). 

I will say, that I was extremely impressed with Meryl Streep.  At one point close to the end of the film she performs a solo musical piece to 'The Winner Takes it All'.  I don't know that I have actually seen someone actually act that well while singing.

There are a few places that are cheezy, but the performers know it and they embrace the cheeze, turning it into just good fun. 

If you can't stand musicals, probably not the film for you, but I'd give it a look.