I seem to remember the RS:X's using them and the Mistral OD's as well. I'd be inclined to reverse the last purchase to provide a better lead and cleating option to the one shown in the photo. I used adjustable outhauls on all my race sails but never an adjustable downhaul. I tried to engineer one once but the loads on the Formula sails were to much to adjust on the water (for me anyway) and it was just another complication for little gain.
I race a Starboard Phantom 380 and race on the UKWA circuit in the UK and we use them all the time on raceboards to tune the sail for upwind and downwind or to simply put on more downhaul if the wind picks up in the middle of a race. Modern Raceboard sails have a massive adjustment range on both downhaul and outhaul so using and adjustable downhaul and outhaul enables you to get the best performance out of the sail
Personally i use Holt Allen blocks that were originally designed to be used on a Laser Dinghy and these work great 
