Despite being the weakest Pixar film to date, it's not actually as bad as most people make it out to be. Mater in the spotlight, while it has it's moments, can get a bit tiring at times. The movie seems to try and have heart, but it just doesn't come out the way the first film had with this film being too frantic and fast paced with it trying to be an action/spy movie.
For everything I really like about the Cars films is the world building the creators have to create for these automobile characters. It's a lot of fun to look at and see how the characters interact with the world around them.
For all it's worth, I think it's still an enjoyable movies, but if you weren't a fan of the Cars franchise to begin with, then maybe this is not the movie that will win you over.