Um. Let's put it this way: if you don't like musicals and you aren't in love with ABBA, watching this film won't change anything. I'm guessing you'll even like them both less.
It's basically, mostly middle aged, random actors making complete asses of themselves. Imagine Meryl Streep and her 50 year old 'girlfriends' singing 'dancing queen.. only seventeen' like they really mean it. I'm guessing both Meryl Streep and Pierce Brosnan were bored the day they recieved an offer for 'Mamma Mia!'. Yup, Pierce Brosnan... he absolutely made me laugh. I thought I'd never live to see the day 007 would put on a one piece suit resembling an ABBA halloween costume - to see this, however, you have to live through the whole movie, because the 'best' is saved for last.
I like some musicals (for instance: Sweeney Todd, Chicago), but I have to admit, Mamma Mia was too much. The singing wasn't that good, either. But, if you're a musical movie freak, then this may be your guilty pleasure. Very humiliating - guilty pleasure. ;]