I've recently purchased a UCF21 Celsior with air suspension, replacing a 80k miles LS400. OK, I know I'll be up for an expensive bill at some point but wow, I'm sold. No diving on hard breaking, much less roll and wallow around corners than shocks/springs and of course the smoother ride on shocking London streets. I can fly around those corners at greater speed utilising the surprising amount of grip the car has as the car has a greater propensity to stay level than on steel springs.
I priced up replacement OEM standard shocks/springs/spring seats/top mounts and it came to about £400 per corner, and the air shocks are £700, so I'll probably stick with air when the time comes for the sake of £1200. The enhancement to the ride and handling of the car is well worth it IMO, and the car is a keeper.