If you have a reasonable attention span, the campaign is really enjoyable. Not genre defining or any such hype, but enjoyable. FPS and vehicles, there aren't that many titles that try to do that properly, let alone do it well. I think FC4 does it really well. There are a few little glitches with the NPC's from time to time, but nothing game breaking
The mechanics are great and the visuals a really big step up from the last installment. With that, all the side missions and the user generated content, this game is a real keeper if you like simply doing your own thang.
There is of course the PvP as well, but that didn't interest me in the slightest, so I haven't bothered to try it.