The WooLee Winder is particularly nice for even plying, as you don't need to stop to move the yarn guide as you do on manual flyers. The tradeoff is a very slightly heavier, less responsive feel due to the inertia and friction of the level-wind mechanism. It is very important that you keep the mechanism clean and properly lubricated for best results. Not a big deal, just beware that it's important.
The flyer speed ranges from zero to about 1400 RPM; a bit slower than the other flyers due to friction and wind resistance.