The talking dog scenes played a major role in keeping the movie from being a...well, a dog.

Rating: Null/10 (5 July 2002)

The wicked alien, Serleena (Lara Flynn Boyle), lands on Earth to recover a powerful light device with which she intends to destroy Laranna, Queen of the Universe (Paige Brooks) and take over the whole works. Since the MiB mission is alien control, they can't allow that. What's more, if that device doesn't leave Earth by a time certain, Earth will be destroyed!! And only former agent Kay (Tommy Lee Jones) knows where it is.
Needing his old partner back, agent Jay (Will Smith) gets rid of his incompetent new partner, Tee (Patrick Warburton). Jay seeks out Kay who has been neuralized (memory wiped clean of any MiB traces) and turned out to pasture in a small Massachusetts town, where he is the Postmaster. Together again, they set out to save the world.
After a reasonably clever introduction, the movie falls on its face, ker-thud, in the sequences where Jay gets rid of his partner and saves a subway train from being entirely devoured by a huge subway-dwelling alien (it's set in Manhattan). We took in a 4th of July 11:40am showing, and there was a good turnout. Alas, you could feel the anticipation congeal, as the subway sequence labored on to its conclusion, which, thank goodness, consisted of a pretty good sight gag.
Things picked up, however, when a talking dog showed up, and the movie won us back, but it had burnt up a significant chunk of good will by then.

I found all the characters to be either interesting or engaging, and often both. Even the animated characters, which I'm not always that fond of, were a lot of fun.
The plot was just about right, consistent, not too complicated to follow, and humane enough to allow for a touch of romance (provided by Rosario Dawson). It accomplished what it was supposed to do, which was to provide a structure on which to hang jokes. The jokes weren't bad, some were really clever---individually---but they didn't seem to find a rhythm, and so, in concert, fell somewhat short of orgiastic mirth. Except for the dog scenes, which played a major role in keeping the movie from being a...well, a dog.

I can hear Will Smith, even now, bemoaning "A dog stole my movie!"
Lara Flynn Boyle was straight-on deadpan as the wicked alien, and could have been hilarious. But, aside from her first entrance, they gave her nothing funny to do or say. Smith and Jones work well together, and I wouldn't mind an MiB III, if they could find a script with just a teensy bit more wit to it.
Did I mention the dog was great?
Worth seeing at matinee prices, but don't set your expectations too high, given those flat early-on sequences.