In my view of the world, yarn can be divided into three major types. First, there's the cheapo yarns. These are usually made of acrylic and sold primarily at large discount and craft stores. I am a fiber snob, but I do think cheapo yarns have their uses. I used to knit with them myself when I couldn't afford anything else and there were no yarn stores for many miles around. Cheapo yarns may sometimes be appropriate for kids' things because they're machine-washable and tend to wear well. Now the craft and discount stores are starting to carry nicer yarns, but they still have the cheapo stuff.
Second, there are the basic yarns. I don't mean basic in quality or price, because many of the yarns in this category are very nice and very expensive. Basic yarns are yarns you could knit a sweater with. Lamb's Pride is a major player in this category, but there are many, many others. Some of my favorites are Manos, Noro, Blue Sky, and Elsabeth Lavold.
Third, there are novelty or frou-frou yarns. These are the eyelash yarns, the fake furs, the odd glittery things, the railroad ribbons, and so forth. Crystal Palace and Berroco are two companies that manufacture a lot of yarns in this category. They have their uses, and I do buy them sometimes. But you're not going to knit a sweater out of them. Unfortunately, I think a lot of yarn stores concentrate too heavily on this category of yarn. They may be more glamourous on the shelf, but one can knit only so many frilly scarves. I think maybe these yarns especially appeal to new knitters who do make mainly scarves or ponchos.
All three types of yarn have value, but when I look at my stash, I find that nearly all of it consists of the second type. And my favorite yarns stores are those that carry mostly basics as well. When I'm going to spend many hours on a project, I want it to be something that will continue to look beautiful several years from now, and not dated.