I use PreFlight parking at both BWI and PHL. Join their frequent parking club and you can earn points towards free parking. It doesn't take long to get free parking. When you enter the lot, they tell you exactly which section to park in, and the shuttle knows you going there. We never have to wait more than 2 or 3 minutes for a shuttle. Plus, the facility at BWI is fairly new and huge. AND, if you are member of their frequent parking club, they will take you even if they are full up. They also have on-line coupons for parking discounts.

I am not an employee of PreFlight, so please don't think this is the reason I sound so positive, but I've never had a problem with them, and only wish they were at EWR and JFK.