Part of its success is brilliant casting. When you think of a younger Tommy Lee Jones, Josh Brolin does not come to mind. But his Agent K is uncanny. He's got Jones's mannerisms down pat, but there's a levity and energy to his performance that reverses the passage of time. Michael Stuhlbarg (A Serious Man) plays Griffin, a fifth-dimensional being. He's constantly babbling about which future he's in and the paths people might take. This could be annoying, but Stuhlbarg (in his terrible woolen cap) sells it. He's dorkily endearing. Jermaine Clement, of Flight of the Conchords, is terrific as Boris the Animal. ("It's just Boris.") He's unrecognizable except for his deep voice, which is perfect for Boris's menacing proclamations.
Emma Thompson and Alice Eve also join the cast as older and younger versions of Agent O. Thompson does a fabulous job as always and Eve is fine in her few scenes. But the film didn't quite earn K and O's semi-tragic never romantic relationship. It's the problem of a series where each movie is produced without a plan for continuity. K already had one wonderfully done failed relationship. This one feels obligatory, like you can't have a summer blockbuster without a love interest. O would have been fine as a fellow agent who just cared for K as a long-time colleague and friend. (Or perhaps I'm just frustrated that at this point in the franchise all of the female good guys are love interests.)
There are a couple of standout scenes. There's a funeral for Zed, played in the first two films by Rip Torn. Obviously Torn couldn't be in this film because of his legal troubles. K's eulogy, paraphrased, "He never told me anything and after decades I know nothing about his personal life," is perhaps the greatest compliment one man in black can give another and also hilariously brusque. There's Agent J being hassled by cops in the past. There's also Bill Hader's terrific cameo as Andy Warhol, unfortunately spoiled by the trailer.
If you didn't like Men in Black, then I'd avoid Men in Black 3. (Common sense, really.) But if you did enjoy it, the new film might be a pleasant surprise.