The game is a "manage your criminal organization" strategy game, with turn-based combat missions and a setting in 1920's Atlantic City.
First of all, the mood and atmosphere is excellent. The graphics and music are really good. The voice-acting is fairly decent. Although the story is rather stereotypical, it isn't bad by any means.
About the gameplay, I like to compare it to Xcom Enemy Unknown, because they both combine a management game with turn-based combat missions. Xcom EU won several prices last year, including strategy game of the year, and got generally high reviews.
From what I heard, the reason Omerta got bad criticism was mostly due to bugs and the management aspect being fairly simple. Funny enough, I feel the same applies easily to Xcom EU: base management is totally simplified compared to classic Xcom, and multiplayer is so bugged it's hardly playable. The difference is Xcom patches only seemed to scratch the surface, while I've recently played a lot of Omerta and haven't encountered a single bug.
And what about the turn-based combat ? Well, I actually prefer Omerta, I really feel they did a great job on that. Especially "line of sight", which is a serious issue in XCom, is well executed in Omerta. Everything also seems to be nicely balanced. I'm not sure how active the multiplayer community is, I'm afraid it's pretty dead thanks to the bad publicity the game received.
Bugs: no big issues I'm aware of
Management part : fun, but pretty casual, not that much replay value
Turn-based strategy missions : excellent
Gangster setting : great
Maybe I'm biased because I really like both the genre (management + turn-based strategy) and the setting (gangsters). So it probably depends what you're looking for. Biggest issue seems to me the limited replay value.