I use mine for the creepier sort of knit fabrics too, some of the jerseys which wriggle are better tamed with a walking foot. Otherwise, as you, I use it for velvets, and other tricky fabrics. (try stretch velvet, so much better with the WF). Mine came with my machine but I think I would have bought one anyway. It's a bit of a fiddle getting it on and off the machine (Bernina) but worth the effort. HTH x