Much better than it has any business being, given how uninspired the second installment was. One can only reach the conclusion that Tommy Lee Jones was pissed off for some reason; either he didn't want to appear in it, hence the bookending, or he was cheesed-off at getting shunted to the sidelines. Smith's line about him looking over 100 isn't far off; the man always had a sand-blasted visage, but there's something slightly fragile about him now.
The film doesn't quite find its groove until Smith travels back to the '60s and meets up with James Brolin's note-perfect younger Agent K. The time travel plot is as open to dissection of the (lack of) logic of its rules as you'd expect, but at least it's asking its audience to pay attention. This is most evident in Michael Stuhlberg's scene-stealing (and dialogue-heavy) performance as fifth-dimensional being Griffin, who exists aware of the multiplicity of possible futures at any given point.
Smith is as reliably charismatic as ever, and is given some decent material in response to racist attitudes (more could have been made of this, as the theme is touched upon much more engagingly than in the po-faced and earnest The Help). Jermaine Clement is amusing as Boris the Animal, particularly his RP present day version. Best of all is Bill Hader's take on Andy Warhol as a man in black who loathes his undercover role. Less sure about Agent O; Emma Thompson is supposed to be in her 60s? And Alice Eve isn't even vaguely like her.