I became a fan of the Superior pre-wound bobbins when I had access to my friend's long arm. We couldn't wind a decent bobbin on it to save our life. It's funny how our perspective changes, I went from the "what's the big deal about winding bobbins" to being all about using the pre-wounds -- only now I can't get them to fit my existing machines!
I order most of my thread directly from Superior, and would always throw in a couple of the sale bobbins. Typically they were random colors but I figured what the heck, a project would come up and with embroidery I would think they will.
I don't do embroidery, but if I did, I would want a big box of prewound bobbins to use. My current machine has a proprietary extra large bobbin and that's nice, but those bobbins are expensive. It's also pretty easy to wind bobbins and load them, but I'd still rather just have a dedicated already wound bobbin supply.