Almost all of Pixar's films are about growing up in one way or another. Some films hit this more on the head than others, and those tend to be my favorites. Finding Nemo, Monsters Inc, and Toy Story each come to mind as standouts. Inside Out follows in these films' footsteps, and is an amazing film of which Pixar should be very proud.
I love the Toy Story movies, but it kind of kills me that Toy Story 4 is happening. When your original films are this good, why or why do they need to bring back that series? Money, I know, but Inside Out made $90 million domestically in its opening weekend. And not just any weekend, it was going up against Jurassic World, which has had the biggest opening couple weeks of all time! It was the second biggest Pixar weekend ever, and is getting marvelous reviews. The fact that Pixar could make a weird movie about emotions, has a female protagonist (a couple female leads, actually), and doesn't rely on crude cartoon humor to win over kids and adults is amazing.
If you have not seen Inside Out, you should go see it. If you don't like the trailers and think it looks boring, you should go see it. If you have even a sliver of a memory of childhood left, you should go see this film. If you have a kid and need to explain their emotions, go see this film. It's not PIXAR'S GREATEST FILM EVER, but it holds up very nicely against the studio's very best work, and of course is required viewing if you are a fan of their work.