The hacking abilities you gain as you level up are a fresh addition to the usual gameplay of this type of games, and the more traditional driving and combat mechanics are quite fun and well implemented. The main character and story aren't that remarkable, but the missions were varied and interesting enough and, although I didn't care much for some of the side activities, I really liked most of the "investigation" ones that involved looking for things around the city and solving simple puzzles based on hacking connected devices around a location. Another thing I really liked is the map, Chicago and its surrounding area look great and believable and include a nice diversity of environments. I wasn't sure at first, but I ended up having a lot of fun playing this game.
More info: https://en.wikipedia.org/wiki/Watch_Dogs