Mamma Mia was great! It was far better than I expected. I ended up seeing it twice, and I don't think I've seen a movie twice at a cinema since Waking Life in 2001. The Greek islands it was filmed on are beautiful and this was skillfully used as a backdrop for the movie. ABBA songs were skillfully integrated into the plot and sung quite well IMHO. I even enjoyed the plot even though I'm not generally that interested in romantic plots. Perhaps the best part overall is that taken together it all made the movie full of a sort of happy energy.
It seemed like others at the cinema also really liked it. Both times there was applause after the movie and that is quite rare. The second time there were some people dancing during the credits.