Does anybody actually enjoy descending at speed on fixed?
Anything over around 175 rpm gives me serious concerns about my mortality; it isn't fun (for me, although if you get a kick out of danger, or can't resist a challenge, you may feel differently). Substitute your own limit if 175 isn't fast enough. I like lowish gears so high cadences are familiar, and OK up to a point ( in my case things start getting uncomfortable at around 150 rpm).
To enjoy super-fast downhill spinning you need to switch off your imagination and don't worry about a chain unshipping, a pedal grounding, or just getting the line through a corner slightly wrong.
I'm inclined to agree with zigzag that ss is safer, and certainly faster downhill ... but, but, but when the gear is just right, riding mile after mile on fixed at a rock steady cadence feels more efficient than any freewheel (single or otherwise). I think that's part of the "zen" that fixie fans seek.
I'll continue my schizophrenic way running fixed:free in the ratio 60:40. Both feel right depending on (1) the terrain (ss feels best on hilly rides), and perhaps more importantly (2) how I'm feeling ( fixed feels more of a challenge, largely because of fast downhills, but it's great when I'm up for it).