On their own, they are tuneless wankery with no redeeming musical values but in the context of Slayer's often very tight and rigid songs, they give a sense of craziness and spontaniety to the proceedings. I also like the way he and Hanneman shifted the Judas Priest- alternating solos approach to the left while still retaining the dynamic. Basically, in JP, Glenn Tipton played the melodic, classical-inspired solos wheras KK Downing spazzed out with the whammy-bar and played more in a post-Hendrix (minus the blues) style. In Slayer, Hanneman pretty much played like Downing and King like a mongoloid but you still get the contrast.

I should also mention though that those tuneless solos became a cliche in more extreme thrash and early death metal as well so the concept has pretty much been played out for 20 years or so; it doesn't have much left to offer and most lead-guitarists in the at least more notable metal-bands seem to have abandoned it, at least as the dominate lead-style.