Was it Pierre Boulez who took a Llama up to FL460 in the early 70s? The story I saw said it had been stripped down to save weight. The canopy entirely frosted over in cold. When he eventually lowered the collective the engine (never tested at that height) flamed out, and with no relight system he started the world's highest ever autorotation. Of course he couldn't see anything, and tried to keep straight by reference to the sun. He knew it hadn't worked when the sun was level with the cockpit floor, but he pulled out of it, the frost went at lower altitude and he landed successfully. Unless anyone else knows any better, and it wouldn't surprise me.
Most posters have mentioned how scary it is. OK, neophyte's question time, apart from heli pilots' natural tendency to worry when above jumping-down distance, what is it that worries you? Controls near their stops? Unexpected divergences?