This game exceeded my expectations. When I first heard about the Syndicate reboot being an FPS (and saw the fairly lacklustre trailer), I thought it was a bit of a waste of a fantastic franchise.
I bought this recently, after some big price drops, and I was impressed at how much better the game was than I thought it would be, and how much more original it was. The single-player campaign isn't wildly original, but there are enough unique gameplay features and creative weapons that it feels like Syndicate has something fresh to offer a very over-saturated genre.
I think a main selling point is the large number of co-op levels. The mission-based co-op gameplay is unlike what anyone else is doing (it's more objective-based than Uncharted or Call Of Duty's co-op modes), and it's a refreshing way to play an FPS. The co-op has more in common with the original game, since you're generally operating in a squad of 4 with a set objective.
The game keeps enough links to the original that you feel like it's set in the same universe, involving the same companies, but is modernized enough that it doesn't seem dated. Overall, this game didn't quite get the credit it deserved; it's not just another stock shooter.