Article Pract has a great selection of yarns. It's just that you have to know what you are doing with the yarn that you buy there. All the sales clerks have attitudes!! They are either too busy to help you with knitting advice or questions, or they act like you are going to sue them if they tell you the wrong information. Instead of showing me how to do something, they often refer me to look online. (but they don't tell me the name of the website!)
If you can, I would recommend STASH in Berkeley. Everyone who works there is friendly, helpful, and will spend time to answer your knitting questions specifically.

Article Pract is conveniently located right off the freeway and has a pretty good selection of yarns. I only shopped here a couple of times before. They also sell a nice selection of needles, patterns, books, accessories and handmade items. They also have classes for beginners and advanced knitters.

Article Pract has a good selection and reasonable prices. They carry plain workhorse yarns as well as fancy novelty yarns. The store set up is cozy but roomy and there is room to sit and browse through patterns. The salespeople are helpful and non-aggressive. However, the store could use more less-expesive yarns, most of their yarns are of very high quality and priced accordingly.

PROS: good selection, location, customer service
CONS: higher-end