I switched to Dvorak 2 years ago. Before that I had typed Qwerty for about 20 years. I feel that I have a good sense of the strengths and weaknesses of both. On paper, Dvorak looks great and Qwerty looks terrible. But Dvorak isn't without its warts, and Qwerty isn't all that bad for those who don't experience discomfort from it.
In my experience, Dvorak relieved the pain in my hands and wrists (although the right pinky took some time to stregthen). My hands and fingers would tire very easily when typing Qwerty.
As for speed, I feel they are roughly equivalent, although my speed has increased approx 10 wpm. I know some very fast Qwerty typists (~160 wpm), and some very fast Dvorak typists (~165 wpm). As for me, I peaked at about 95 wpm.
As a side note, Dvorak isn't the only alternative layout that I have worked with. I also spent some time on modified version of the carpalx BULPKM layout and stuck with it until I was up to about 60 wpm. That layout was extremely comfortable to type on (noticeably better than even Dvorak), but it was apparent that comfort was the goal as opposed to speed.