I've used it. If you try to spin it alone, it will make a very, very weak yarn - I don't really recommend it, although it can be done. My favorite way to use it is to card it in with a short stapled wool or alpaca - it works nicely with the shorter fibers of huacaya alpaca. I typically lay out a layer of alpaca on the card, then spread a handful of the (deseeded) milkweed fluff, then another layer of alpaca to help hold the fluff in long
enough to blend - otherwise it tends to get caught in the teeth of the active card. Then card the fiber several times until the milkweed is evenly blended.
The fiber is not going to be even - the milkweed fluff has a small lump at the base, where individual threads are held together in a poof, and the lump does not go away using this method. So the resulting yarn is a little nubby - but also has added softness and shine.
Of
course, you can stuff a coat, pillow, or quilt with milkweed down - it just takes a lot of gathering to get enough!