I can't really tell you for sure, as I'm not the most experienced knitter, but I know I've bought some yarns at the craft stores and have been happy with them. I've stayed away from things that look like really low end yarns. For something like a scarf, that I'll probably be tired of in a few years and that won't get a lot of hard use, I figure the less expensive craft store yarns are fine. I did buy some expensive high-end yarn from a yarn shop today for a poncho but I intend to wear it awhile and often, so I figured it was worth it... plus I couldn't find a less expensive yard that I liked as well.

I'll be interested to see if others have had bad experiences with the craft store yarns, but so far they've worked fine for me.

I went to Joann's to look at ribbon yarns. Like the more expensive ones, their stuff was 100% nylon. The difference that I could see was the dye. The more expensive ribbon seemed richer and more homemade (that being a good thing.