RockShox started developing their air-sprung downhill forks in 1999 but despite winning several World Cup rounds since then they've only just been released in production form.
The two big advantages of the air spring are easy spring rate tuning and low weight - this is the lightest production DH fork around, which marks it out as a great race fork or a lightweight freeride/extreme XC fork.
The action is as smooth and plush as most coil forks straight from the box and a slight increase in spring rate as it goes through its travel helps to control big drops. The Motion Control damping is impressively consistent too, only choking slightly if you overflow the cartridge by hitting several serious drops in quick succession. Both rebound and compression are simple to adjust externally and the high speed compression damping can be tweaked internally. The Floodgate platform control can also be set anywhere from full climbing lock to a barely noticeable 'skin' of stability via an external knob.
We've had no reliability problems with RockShox' other air-sprung or Motion Control-equipped forks so we've no worries about the WC. It can get bullied off line through big rock sections, but you'll quickly learn to compensate, which leaves only the high price and fixed travel as downsides.