I am a touch typist and type around 70-90 WPM on a qwerty keyboard. I hadn't improved in years (even though I trained using typing programs). So I figured I'd give Dvorak a try.

After about 6 weeks I was up to around 75 WPM on Dvorak, but I stopped improving after that. I kept at it for another month or two, but never got back to my old qwerty speed.

Furthermore, the worst thing was that I started to get RSI like symptoms in my right hand from the Dvorak setup, something which I never ever had before, even with very extended periods of very intensive qwerty keyboard usage. So I switched back. Fortunately it took hardly any effort to get back to my old speed and the RSI symptoms disappeared.