In the control of a professional driver, non-ABS brakes will usually stop faster, given a good road surface. Given a bad road surface, such as loose gravel or sand or unpacked snow, ABS brakes can be dangerous for any driver. Loose surfaces can be stopped in best when allowed to get pushed forward in front of the tires, which is letting the tires dig into the surface and create a chock in front. However, ABS will not allow this to happen, keeping the tires rolling over the loose surface.

With that said, for the "vast" majority of drivers, ABS brakes will prevent far more accidents than non-ABS brakes. On the other hand, in pre-ABS days, drivers were taught to pump the brakes in skidding situations (never advisable with ABS brakes), but I don't believe drivers today are properly taught how to avoid any need for hard braking on loose surfaces where ABS brakes can prevent quick stops.

-- Alan