improvisation does not work for much of metal, because the genre relies so heavily on precise arrangements and a general feeling of constricting "tightness." that said, there have been many guitar solos in metal over the years which are both improvised and worthy of camparison to the jazz tradition.
to say that improv "sucks" or that it automatically leads to jam-band bullshit like dave matthews is to paint with way too broad a brush. what the hippie jam-bands do is to solo over repetitive riffs in the middle of a structured song for ridiculously extended periods of time. when masterful jazz musicians improvise, they can either alter the parameters of a given piece to create interesting juxtapositions or interplay, or they can just weave the piece from whole cloth. either way, they're composing on the fly, and that's something.
not that i'm expecting anybody on this board to just start listening to improvised music...