1. I have them and personally don't care for them. I prefer metal needles (or occasionally wood) and thin, flexible cable. I own multiple Knitpicks Options nickel plated and several harmony needles. Those are all I use now. That being said, many people do like them a lotto it depends on what you like.
2. I was going to say that whether to purchase them would be dependent on which size was missing, but then again...for only $8 you can hardly go wrong. I don't know. Since I don't like them I'm not the one to say really.