Yeah, a few of us use them up here in Canada, though they're not allowed in the US TCS. They're used mainly for outdoors where braking isn't necessary and there's more gradual turning, though it's sometimes used indoors as well.
All the free turning provided by a loose gear diff, but no wheelspin if a wheel lifts like a tightened diff.. the car just rockets out of the corners.
It does take some decent driving skills and timed turning to use it as there's no braking and one late turn can mess you up, but mastering it can get you much better laptimes.
It can be tricky getting gridded up in a main, though, especially on asphalt that is on a slight grade downhill..
