I watched this on a lark and was pleasantly surprised. I think critics worth their salt will mostly have something to say about the third act because it is bittersweet and not wrapped up in a nice bow. The movie is flawed in the same way the character is and I think for the most part that was intentional.
Although the overarching villain and his schemes were a bit formulaic, the period piece humor, (Andy Warhol etc...) Josh Brolin's brilliant impression of Tommy Lee Jones and the great plot twist tie-in at the end raises the movie to a C+ or B status. If you are a true cinephile I would say you only need to buy the 2nd MIB for collecting purposes. 1st and 3rd are by far the best. The third is pretty good but it is lacking certain magic that the first one had.
The funniest parts in the movie were ironically the parts with Kevin which were blatantly sexist and there is no attempted balance...which in itself shouldn't bother anyone outright (the original had some of this), however, the fact that all the actresses did interviews denouncing sexism just makes it hypocritical and weird, furthermore none of it felt like it was in fun spirits, but highly spiteful. The villain is not fully fleshed out, the supernatural lacks mystery, and where the humour in the original felt like something one might actually run into in real life (like the office, or parks and rec), none of these characters personalities felt real. It is really important to ground the characters in a film that is already flirting with the suspension of disbelief. This is why the snide and pithy remarks of Venkman in the original were so great. Honestly, I blame the script mostly. Melissa McCarthy and Kristin Wiig have already pulled this off in Bridesmaids so I was hoping for more.
Artistic visuals spoiled by odd pacing, script, and storyline. All of the redeeming qualities were completely ruined by the one scene with Sam Jackson that should never have been made under any circumstance. Just because one can, does not mean one should. The most impressive performance to me was Jennifer Jason Leigh.