Fox's Float has been around for a while now, progressively increasing travel to the current 130mm long travel XC standard and providing a benchmark for fluid, big terrain performance in the process.
The 'XTT' takes the basic Float architecture and adds Fox's proprietary Terra Logic - locked out until you hit a bump - damping control from their 'X' race forks. The 'Trail Tune' auto start threshold is fully tuneable though, and in its minimal settings there's no obvious delay in fork reaction. Even with enough 'TT' to steady the fork when braking or climbing hard, there's enough free movement to ensure front wheel traction and stop vibration fatigue. The result is a fit and forget fork that works really well in tandem with similarly platform shock equipped - or just firm feeling - bikes. A travel adjustable TALAS version is also available for £649. Fox reliability is generally good if you look after them and the only obvious downside that we can think of is the rather high price.