I work at a local shop and several...meaning more than 3 folks have purchased and do not like them. They say they are a waste of money. It doesn't wind as tightly has a machine bobbin winder would and it seems to break. The owner of the shop when she was in Quilt Market in the fall look at the vendor of these new notions and even the vendor couldn't get the winder to work right.
If you are using a 50% off coupon and basically trying it for $10, it might be worth the risk. I personally would not get one.