Experience was OK. The two biggest problems were that, as other people have noted, the wait was very long (over an hour on a Sunday afternoon in high season) and their section of the garage is difficult to find. Pro tip - ignore the sign on Level 2 of the Sky Harbor rental center that says "Advantage" in great big letters - you need to go to Level 1, which is the Avis level, and take a long walk around the garage to get to the low-rent counter where Advantage and the other cheapie rentals live.
I'm willing to accept those inconveniences to pay half of what I'd pay at Hertz. Service at the rental counter and in the garage was friendly and straightforward if not quick. The compact car I rented was perfectly fine. Nobody tried to upsell me any more than they do at Hertz or Budget. I paid in advance through Advantage's website and didn't come up against any hidden charges.
So, in short, you get hit with some inconveniences in exchange for saving a few bucks, but I didn't see anything unethical, sneaky, or unfair.