Joe Pilot - reports and tests of Vortex Ring suggest that it is far from stable in the fully developed state. Apparently a bubble of higher pressure air forms and then bursts regularly but in unpredictable directions giving random pitching and rolling. Additionally, the disturbed airflow from the main rotor plays havoc with the flow through the tail rotor causing yaw fluctuations.
Stable? - No, A fast way to get down? - yessireeebob!
SPS - the only recoveries are 1. A lot of forward cyclic to gain speed then apply power once out of the dirty air, or 2. Lower the lever to enter autorotation - but you need a lot of height and may overspeed the Nr!
Lu - The BERP blades have a very powerful tip vortices, just like a swept fixed wing - the tip losses were so high in the hover due to Blade Vortex Interaction (BVI) that the had to add the anhedral "droop" tip to spit the vortex out of the way before the next blade came along.