I had almost never trusted off-property parking businesses after a few bad experiences with the logistics in the past. However, The Parking Spot proved to be a great option for our last trip. I'm pretty sure they got us to terminal 4 much faster than the airport's own shuttles.
Two important notes about pricing:
1. Always get a coupon, as I've learned you can almost always find one.
2. They charge a flat daily rate, so do the math to see if you're better off with daily + hourly somewhere else or just going with the flat daily rate.