Gangster Squad is entertaining as hell. It's also a total mess. Seriously, it's like watching a live-action cartoon with an A-list cast trying so hard to make this dialogue credible, but there's nothing they can do. It's a cartoon. Over the top, hammy dialogue, crazy shootouts with no consequence. If you don't shut your brain off within the first 20 minutes of this movie then you're doing something wrong. It's a shame because the plot is based off of the real life gangster Mickey Cohen who was the ruler of LA for a time, and the source material has endless potential. But the faster you can accept that that's not the kind of movie this is, the quicker you can appreciate it for its camp value.
Some things that stand out are Sean Penn's ridiculously over the top performance. Not that you can fault him for it. The part is written as a stereotypical gangster with temper issues and the script throws him these cheesy one-liners that wouldn't sound authentic coming from anyone's mouth. But to his credit, he's fun to watch. As is Brolin who plays the leader of the squad, and probably the most "human" character of them all. And there's Ryan Gosling is doing a really weird but oddly fitting voice for this time period with his natural charismatic charm. The supporting cast is good as well but they practically nothing to do. There's no pathos anywhere - it's all overblown bombast, and if you're okay with that, then I think you'll get a kick out of Gangster Squad.
If you're expecting LA Confidential 2, stay the hell away.