I found the story campaign part of the game pretty enjoyable - well presented, the objectives gave it a bit of momentum, and I liked the turn based combat. There's definitely room for improvement in a number of areas, but all in all I thought it was pretty good.
However, once completed I felt zero inclination to play through it again. So that leaves Multiplayer and Sandbox mode for replayability.
Multiplayer was actually fairly fun, but you only have 2 vs maps and 2 co-op maps, so it gets old fairly fast. And the lack of in-game chat is a real problem when trying to play co-op.
Sandbox, eh, I just don't think the game works for sandbox at all. There's nothing to compete against, the world barely reacts to you, and it never changes of its own volition. So it doesn't take too long until you've bought out all the independent businesses, raided everything you can't buy out, and well, then there's just nothing left to do - no further expansion, no new challenges arising. It doesn't have the sort of living world that some sandbox games have where the world's constantly changing and other agents are acting in it with emergent stories etc; it's just you and minor characterless NPCs who only ever do anything in direct reaction to you, and those reactions are very limited. It also doesn't really have the visual aspect of e.g. a city builder game where you can see your city/empire/whatever building up and expanding. And finally it has a really low ceiling on exactly how much you can expand before you've filled the map and cleared everything of interest (the small maps don't help there, but the way income tends to snowball I don't think larger maps would really help).
Overall I consider it a decent, but also disappointing game. I'd say it's worth buying for the campaign, but only at a fair discount (30-50%). And definitely not something to go into with strong expectations/desires for an update of existing gangster games (which I get the impression is a big part of many people's extreme hostility towards the game).
There's a demo for those unsure though, and it's fairly representative of the rest of the game (perhaps a little too much, in that besides new gang members there's not an awful lot to unlock in the full game that you can't also get in the demo).