Suffers from my a loose plot, but the major(and minor) actors deliver.

Rating: 8/10 (25 May 2012)

If I'm allowed to give only one piece of advice to the writers in Hollywood, and for what it's worth, I would say that writers should stay away from time travel. As my proof, I would present to them the MiB 3, a movie that delivered on most accounts but was thoroughly let down because of the loose plot. Time travel has been done well by some movies only because they dwell on it, taking a considerable amount of time to plug the holes and shoring up the loose ends. In this movie, however, time travel is just a convenient plot device another dimension to Agent J and Agent K, both as individuals and as partners. It is especially superfluous when time travel is used as combat strategy in an action sequence. In short, the whole time travel part was thoroughly needless, and flashbacks--or something else--would've done everything the writers intended, but without leaving the more plot holes than a block of Swiss cheese left out in a firing range.
If one can suffer me to dwell on the negatives bit longer, I can relate another major problem with the movie: pacing. Will Smith and Tommy Lee Jones, as Agent J and Agent K, have some actual chemistry, and there some moments that really did get profoundly emotional. It is quite plausible, expected even, that J as K's partner for 14 years would deeply suffer from the loss of his partner. The movie, however, expects us to play out at that emotional drama in our head. During the second and the third acts of the movie, there are scenes that keep pulling at our heartstrings by displaying all the tiny things about K that J missed: eating pie, drinking coffee, listening to country music. Here's when the problem with pacing comes in. Agent J has been barely separated from K for 12 yours! Unless J is a hyper-sensitive, emotionally needy man who cannot go without half a day without pining for his partner like a stricken puppy, he cannot possibly miss K that much. I won't lie and say that I wasn't affected by those very scenes, but at the back of mind, I was relishing those very moments if the movie had managed its time better i.e. if there actually had been a separation between the two agents!
Now, going on to to things that worked: Tommy Lee Jones, Will Smith, and Josh Brolin (as K of 1969) all deliver solid performances. The comedy derived from that man-love relationship is just what you would expect-- and want. There are is an especially great cameo from Bill Hader as Andy Warhol, and there are are numerous other pop culture references creating a certain temporal irony--even racism is light-heatedly (but effectively) tackled. One might wish that the entire movie was comprised of that, but as Prates of the Caribbean: On Stranger Tides showed us, a series of loosely connected comedic set pieces cannot alone carry the movie. MiB 3 knows this, which is why the majority of the movie revolves around the relationship between J and K, and there is enough serious pathos that makes the comedic moments memorable.
Two characters, the movie's main antagonist, Jemaine Clement as Boris the Animal, and Michael Stuhlbarg as Griffin, a fifth dimension being who can perceive an infinite number of realities simultaneously, are two characters that add little to nothing from the movie. They don't necessarily hurt the movie, but if their characters were tweaked a bit-- or in Griffin's case, not added at all--would've have movie all the much better. Boris is an archetypal villain. He is without any remorse or compassion, he carries out assassinations ruthlessly (although he is quite incompetent when it comes to killing an MiB agents), and has a burning desire for vengeance. For all his faults, there aren't any redeemable qualities that emotionally manipulate us at his eventual downfall, but he isn't also quite evil enough that we wish his death with a passion. He's sort of there simply to move the story forward. Most of time he assassinates aliens off-screen, and forgive me for this, but it's hard to sympathize with aliens. A slightly evil Boris, or a slightly less evil Boris would've been better than the cookie cutter Boris that we have in the movie.
Any amount of time that Griffin gets on screen is an absolute waste of time. I know it's hard not to deliver some philosophical perspective when the very secrets of the entire universe are at stake, but pseudo- philosophical perspective delivered by Griffin is very much bland and unimaginative. In the hopes of appearing existential, Griffin spouts a lot of nonsense that you just have take on its face value because there is nothing deeper within. a) When there is an inter-dimensional being who alone has the key to stopping a genocidal race of aliens, why wouldn't it use it on his own plant? b) Why is the physical presence of a "fifth dimensional" being is so important, and why can it be hurt physically when the creature is metaphysical? c) It could very well be irony, but why is the favorite moment in human history of this preternatural being a goddamn baseball game? No matter how much the movie tries to explain this last point, a baseball game--or any sporting event for that matter--seems for too trivial in about 7,000 years of human civilization.
For all the faults that I've mentioned, I gave the movie a score of 8. I chose to dwell on the things that didn't work because there was an agonizing amount of potential in this movie that was all wasted. There was a lot of thought that went into the movie to create the relationship between J and K, the main plot, however, could have been procedurally generated by an algorithm: cookie-cutter villain, convenient plot device, and some pseudo-philosophy to make the movie look deep.