Pixar can do sequels incredibly well (Toy Story 2 & 3), or very badly (Cars 2). This is pretty good; I would have had a good time even if I hadn't been escaping from the oppressive heat this past weekend.
It's actually a prequel to Monsters, Inc.; we get to see Mike's backstory as an earnest kid with a desperate wish to become a "scarer". (Which, since we've seen Monsters, Inc., we know he does.) Surprisingly, this career path requires higher education, and the best to be had in the Monsters universe is … well, see the title.
Somebody pointed out that the plot is a shameless ripoff of Revenge of the Nerds. (And probably a few dozen other college movies contribute their DNA as well.) So lot of the plot is foreseeable: for example, Mike will meet his soon-to-be lifelong friend Sully at MU, but (of course) they don't initially get along well at all, but amusing slapstick incidents keep forcing them together.
So: it's not as insanely wonderful as some Pixar flicks, but kept me chuckling and marvelling at the creativity and imagination of the moviemakers. I may not splurge for the DVD, but (on the other hand) I enjoyed it more than I did Man of Steel.