MIB has come a long way since its origins as an underground comic. I was a little dissapointed with the second film, but the third one was worth the wait. I think it's the best one of the trilogy, and it's definitely worth watching in 3D.
They've continued to stay imaginative with not only the story, but with the aliens themselves. Boris the Animal, the primary antagonist, was brilliantly done. How he functioned, if you will, was out of the ordinary, even for them. But it was good. James Cameron Avatar type interesting. Boris had something, maybe another alien, maybe a part of himself, that lived in his hand, but could come out attack. The thing was a beast.
Will Smith looked younger than he has in years, and he was still cracking jokes just like before. Tommy Lee Jones was as stoic and emotionless as ever. Josh Brolin, who plays the younger version of agent K, was believable. I don't how long it took him, but he had Jones' facial expressions and tones down pat. Andy Warhol was hilarious, as was Grif, an interdimensional being who could see an infinite number of realities, and is constantly commenting as to which one it might be.
Time travel gives the audience some great laughs. From Will Smith being pulled over by racist cops to a portable neuralizer working off of dial-up internet, I was cracking up the whole movie. And the worms playing bagpipes while singing "Amazing Grace" at a funeral--priceless.
Without giving any spoilers, there was more depth to this one than in any of the other films, delving into some motivations that you don't necessarily see coming.. The ending gives you more back story on the characters than I would have imagined. Very intense.
So, in a nutshell, MIB is definitely worth watching. And if you watch it on the big screen, spend a little extra and watch it in 3D. It's not on the level of Avatar, but it's still worth it.