Well, it depends on who you ask and what you are comparing the yarn to. I haven't been very impressed with their yarn myself but lots of people like it. The yarns that I have ordered from them are rather loosely spun and not as robust as some regular brand name yarn. Probably ok if you are not making something that you want to last for a long time. Sometimes as is the case with their lace yarns there are much better yarns available elsewhere that are only slightly more expensive.
I feel the Elann house brands are somewhat superior to Knitpicks. Softer, nicer colors and a little better spun. I also like Brown Sheep yarns and things like Cascade superwash that aren't very expensive either.
If you are trying to get your order to a certain amount for the free shipping, you could consider ordering books and other items. They give a very good discount on books, software, etc.