I think David Mamet is both underrated and overrated.
If you know anything about film, as Mike said, he's one of the few writers in Hollywood whose name is known by everyone. A lot of people proclaim him one of the best dialogue writers ever and that's what I think is extremely overrated. Lately in his career, he tends to churn out verbose scripts with dialogue that may sound smart to pseudo-intellectuals, but is not organic at all. You'd never hear people in real-life give the kind of drawn out speeches some of his characters do. I mean, if you know anything about film, Mamet's name is paraded about as the end-all, be-all for writers. In that respect, he's overrated.
(on a sidenote, I think considering Kaufman's still relatively short career, I'd rate Mamet over Kaufman, but Mamet's in danger of Kaufman's small, but solid and sometimes fantastic, body of work eclipsing his own)
A lot of people think Mamet can't be entertaining, and that's where he's underrated. He's put out plenty of purely entertaining films, with The Untouchables probably being the best example of this.
karasu said:
He's definitely underappreciated.
By the mainstream? Probably so. But Mamet isn't begging for work and he's not a washed up nobody where it counts -- Hollywood.