The smoothest abseil you could ever want, very little rope twist, and capable of abseiling on doubled ropes of 8mm - 11mm. Can also cope with very stiff or thick ropes (up to 13mm)
What's the drawback? They are harder to lock off in quickly, than a Simple (or Stop, of course) and are marginally slower to thread too. The bars seem to individually wear faster than their counterparts in the Simple.
Having two fixed bars, these are pretty hard to 'suicide rig'...despite what some films might suggest! If contemplating 'Big Pits' or caving in the States, where stiff ropes are most likely to be met, then the rack could be jsut the thing. Friction is very easy to vary, so single hangs over 100m are most easily approached with this kit. It is also harder to glaze the rope with a fast abseil BUT THIS IS NOT A CHALLENGE...