People underrate this game based on their experiences with other racing games. No matter how realistic your sim racing game was, chances are you weren't racing realistically in it at all. The AI in Grid is amazing if you give it a chance. I was used to the Gran Turismo "why am I not winning yet I've been racing for over a minute" style, but that doesn't happen in Grid. You generally start in 8th to 12th place and will have to actually overtake the drivers one at a time. The AI drivers actually react to you and as long as you try to race somewhat realistically you'll end up having great fights. If you're used to ramming as hard as you can that great piece of work will be lost on you. I was like that at first too, at the start of every race I would aggressively ram everyone out of the way, which resulted in a broken car 3/4 of the time, so I would restart the race until I got it right, resulting in frustration and me disliking the game. If you can adjust to racing realistically yourself, It becomes a lot of fun.
The game also goes to some effort to keep the cars relatively close together so there's always battles going on. set the AI to one of the hardest difficulties, force yourself to race from the first-person driver view, and then add controls for looking around and racing actually becomes exciting. Turn on pro mode (no race restarts, no flashbacks), and it's even more intense.
Finally, you get to buy a teammate for your racing team, and if you invest in a good one some racing options will open up. Such as, in a 6-race championship I stayed behind him the entire time, keeping cars behind us at bay so he would get enough points to get him a second place in the rankings, while reducing the amount of points the team in second place would get. This earned my team a double victory for the team and a team win overall (which earns you a lot of money)