I've been meaning to take my car here for a while and finally did it today. Last time I stopped by, I didn't have a Costco card (you need it even if you're paying with another card). There are two lines and four machines to pay at. A sign says to pull up to the last one to pay, but since there were so many cars it made sense to just pay at the first you got to. Hang onto your receipt. I waited about 10 minutes in line before I got to the entrance and the guy took my receipt and told me to put the car in neutral with my foot off the brake, standard for any automated car wash. The machines seemed pretty powerful and they did a good job cleaning. For $7.99, it's worth the money, though I wish the wait was shorter and it would be cool if they had vacuums.
Tip: while you're here, I don't recommend filling up gas. Scottsdale Costco is always more expensive than the PV mall location (today the difference was $.10).