oh how I love Preflight! Traveling these days can really suck - it's a hassle getting to and through the airport so I have to thank Preflight for taking some of the pain away. I've had nothing but great experiences with them.
The booth attendants are always friendly and happy. I don't know how they do it, but without fail, no sooner have I gotten out of my car, and the magic bus is magically there! I don't think I've waited for more than 2 minutes ever. They keep the shuttles icy cold in the summer too. My absolute favorite thing about Preflight is the drivers. These guys are not messing around, taking the scenic route to the airport and driving as slow as possible. They're efficient, and they're courteous. Best of all, when you return to Sky Harbor, you won't wait long for a shuttle to show up. I don't know how often they run them, but I've come in at all hours of the day and night without having to wait forever.
Also if you want a car wash, they do an incredible job - the best car wash I've had in PHX. They even vacuumed out my trunk without me asking or charging me more considering that I'd been hauling plants all around in it the weekend before.
Do check the website for coupons and consider joining their frequent parker program for some nice perks.