I wind my own bobbins, but I have longarmer friends who like the pre-wounds. It can be hard to find a good stand-alone bobbin winder if one didn't come with your machine. Also, if you want to use the same thread in the bobbin and on top then you have to choose between buying two of the same color, winding a bunch of bobbins ahead of time, or moving the cone of thread from your longarm to your bobbin winder and back again. I've been told that the pre-wounds also hold more thread, though I don't know that for sure.