Some musicals are alright (Blue Brothers, that one Buffy episode, Dr Horrible, Southpark Movie, Disney films) but these are on the whole male focused musicals, the majority of musicals are effectively "Chick Flicks".
I find random singing and dancing on the whole annoying so some explanation is needed for it (Blues Brothers all the singing is part of the story, Buffy they were cursed by a Demon), or a greater suspencion of disbelief (such as cartoons, webblog themed, etc).
Things like Mama Mia just piss me off as not only is it ABBA (who make my ears bleed) but its also that type of musical where everyone starts singing and dancing as if its perfectly normal thing to do.