Having worked at an Autozone for 3 years during college, I have to say that it is all in who you talk to. Autozone pays just over minimum wage for most of their regualr workers, so they obviously don't attract many ex ASE mechanics. A lot of the time, they have things that you need, they just don't know it. As an example, Rick mentioned copper brake washers, which I know for a fact they have because I have sold them. Also, the clay bar that started this thread, I know for a fact they have those too. Actually they carry two brands, Clay Magic and Mothers.
It's all in who you talk to, and how much they care about their job. Admitedly, we had our fair share of people that knew nothing about cars, but our management (me included) was always there to pick up the slack. I know that a lot of stores don't have nearly as good staff as we did, and I understand why.
There is a trade off for cost versus service. Autozone doesn't always have the best service, but they do usually have the cheapest or close to cheapest prices. And as long as you don't buy the obviously cheap parts (1yr warranty vs. lifetime) then their parts are pretty good and reliable. If you go to NAPA, you will generally get someone that knows a lot more about cars and can answer a lot more questions but you pay for it in the cost of the part too.
Some autozones aren't worth going into, I admit. After college I transfered to an Autozone back near home for the summer while I waited to find a real job (aka college degree). The manager was horrible, and misinformed people all the time about parts we did and didn't have. Somehow he had become a store manager after working for Autozone for only a year, and didn't know enough about the store and our inventory. I had more time experience with Autozone then everyone else that worked at that store combined. I continuosly had to correct the Manager when he misinformed customers. He didn't take this well and took it out on me in scheduling. I got fed up with this and walked out after only a month. 3 years at the other Autozone had been great, never a problem and I learned a lot, but 1 month at this store with this manager and I was fed up.
In the end, I'm not defending Autozone, I know that some of them suck. But I also know that if you find the right one or talk to the right person, you'll get a whole new perspective. You can't expect a lot from a parts house that pays their employees the same as McDonalds, its a flaw in their buisiness model, but as a buisiness they are trying to make as much money as possible, and they are doing a good job of it regardless. Autozone definately has the market share for DIY (do it yourself) parts stores.