I've never been a big fan of the MIB movies, but I thought this one was the best of them. It's a good story and Josh Brolin is great as young K. Will Smith is better in the movie than he seems in the commercials too.
I really liked the story aside from one major complaint (SPOILER ALERT)...
When K is killed in the past and prevented from deploying the arcnet, why didn't the blogon (or whatever)invasion happen right then? Why does it happen 40 years later, conveniently in J's present? This is something all the MIB movies do, which is to have a smaller story but introduce an element of unnecessary global disaster that has nothing to do with the story but gives the movie the illusion of scope. The story would have worked a lot better if J had gone back in time to help K and inadvertently prevented him from deploying the arcnet, thus forcing him to redeem this mistake and stop the invasion. With that one tweek all the other events of the movie could have happened exactly the same and made a lot more sense.
On a lesser note I also didn't think that the events of the story properly explained why K becomes a taciturn, emotionally shutdown zombie for the next 40 years. I'm sure in MIB he sees lots of people get killed all the time. I thought it would be a lot cooler if J got killed and witnessing that made K removed and emotionally withdrawn in future dealings with him. Knowing how his dad died would have helped J in dealing with his absence instead of treating it like some big weird dark secret.
But whatever. All things considered, I thought it was a pretty good movie. I certainly think if you enjoyed the other two movies you should enjoy this one.