What is the advantage of screw drive lenses?

I assume weight and mechanical parts in the lens. Anything else? After seeing a sdm lens for the first time, I love the silence of the focus instead of the grinding zip zip of the screw drives. I know Pentax has some sdm failures, but it seems like everyone else out there has been able to master this. What's the pro's and cons of each, and is Pentax ahead or behind with the screw drives?