Good, nasty, fast-paced violent fun. A nice companion piece (though not as high a quality), to The Untouchables. I didn't mind the occasional slo-mo sequences and yes, the writing on the wall was obvious for some of the (thinly drawn) characters.
But as far as a good, old fashioned shootem-up goes and ideal fodder for a Saturday night, this hits the mark. In a sea of current 'worthy' films (Life of Pi, The Impossible, Les Mis..), Gangster Squad is a top 'bloke' movie, with blood, guts, fisticuffs and a gorgeous Emma Stone strutting her stuff. It won't win any awards, but highly recommended. And Sean Penn is great!