For a film with such an awesome cast, it may seem strange why Gangster Squad got a January release. But, after seeing the finished product, it's perhaps not as big of a surprise. It's a film that has a makings of a great gangster picture, from the actors, the setting, to the stylistic flair, but, in a lot of ways, it falls flat.
The acting ranges from serviceable to poor. It's not readily easy to tell if this is a symptom of a lack of direction, lack of performance depth, or because of dialogue. It's probably a mixture, with Sean Penn being especially disappointing, seeming more comical than nefarious, though his make-up certainly didn't help.
The script is what really lets Gangster Squad down. The dialogue is often too blunt, stilted, and too corny. The film wants to be a neo-noir esque Gangster picture, but has the writing talent of a B action film. The scenes don't flow especially well, and the characters are grossly underwritten, clichéd, or too familiar. The grittiness is simply not seen in the intangibles.
The one positive of Gangster Squad is the style of film-making. The world building is fairly believable, with beautiful cinematography. The action scenes are particularly stylized, and actually look good, though are staged in a bit of a lackluster, self-indulgent manner (the slow-motion is horribly overdone).
Overall, it's not an unwatchable film, but it's certainly disappointing from a standpoint of its cast and seemingly interesting premise.