Kiwee, a member on the fordflex.net forum has full air suspension on his Flex. It was completely custom welded and done up from scratch just about.
He has said on numerous occasions that coilovers potentially ride better, which intrigued me greatly.
Anyway, here is the gorgeous thing
He was the first to do a lot of things with this platform actually. I would have followed suit but my audio system is located exactly where his air system is.
If you want to research more, he's Kilian Weekes, and has been featured in a number of articles on the web and print.
I'd personally install that on my Edge if a) I wanted ride height to remain same despite "cargo" weight, and b) I wanted to lower the Edge to allow easier handicapped access without installing a swiveling chair/stepside.
I was considering it myself; for luxury of raising the car when needed.
However I decided against it when I realized that I wanted to track the car more--air ride has a lot of flex so I'm told.
Of course; now I'm back to the not wanting to track the SHO because frankly it's a HEAVY car and the AWD isn't full-time.
So my hot rod buddies and I are talking about it again. I kinda have an idea that it would quite funny to roll into a place and put the car on frame. HAHAHA.
NO NO; really for me is clearance. I'd like to be able to raise it to stock height in winter and drop it back down during summer.