The Natalie Silk Spindle was designed based on historic Victorian silk spindles in the book Handspindles by Betty Hocheberg. It is a very fast spinning spindle, designed to spin silk and other very smooth, slipper fibers that need twist added quickly. The fast twist helps when spinning silk because of the fiber's slipperiness -- the faster you can get twist into your fibers, the less it will slip apart in the spinning.
Flax would be difficult to spin on the Natalie, in my opinion. That said, it could be done. I mostly pick a spindle with a big whorl to spin flax, so I can get it spinning for a long time -- my preference is not such a fast spin for flax, since I need to wet it as a long spin. A Kundert spindle would be a good spindle for flax, or a Tabachek Deluxe or Plying spindle -- those all have wide whorls, so they have longer spins. And for a long, fast spin, a thigh-roll will really get these spindles going!
(an IM response to an internet inquiry, 26Jan07)