I've enjoyed the game so far. It's humor is cutesy, and something my wife and I can enjoy together while I play through the story.
I enjoy the missions quite a bit, but find exploring the actual city somewhat unrewarding. I think part of the problem is that you can wreak havoc on the city without any consequences, whereas in GTA games when you cause too much trouble you'll have police coming after you. I find myself missing some of the tension of escaping police or causing as much trouble as possible until I either get killed/arrested.
When I play any version of Grand Theft Auto, I find the missions to be uninteresting and sometimes frustrating, but take a lot of joy in driving around and exploring the world/making havoc. I find the opposite true when playing Lego City: Undercover.