For me the best part of the APS circuit books is the fact that they come directly to your door. There's only one stamp store worthy of the name in my entire state. And while stamp shows are great, the timing doesn't always line up for me and attending usually requires a long drive. Some might wonder if the cost per circuit is worth it. Once postage and insurance is totaled the cost per circuit approaches $10 before you purchase a single stamp. For me that price of admission is a trade off I decided is worth it given what I receive in return. I'm glad that the APS and collectors/sellers invest the amount of time/resources they do in exchange for what I imagine is often a marginal return.
Wheatcent, I'm not sure what you collect, but most circuits I've been on contain plenty of stamps for beginners. Yes, you'll see some expensive items, too, but I've seen books filled with nickel and dime stamps. You might want to start off with just one or two circuits to see what you're getting into; probably not a good idea to start off with a half dozen circuits right off the bat.