I have used one. Too simple to install:
http://www.airgunadvice.net/viewtopic.php?t=13340 If you remove the air gauge & block (that the gauge screws into inside the tube) you loose no air volume. I tried it in a .177 Marauder just today 905fps +/- 9fps for 60 + shots off 3000psi. As FT is my thing this would be just fine with me. It is adjustable, simply screw the knob on the front of reg (naturally it must be out of the rifle) to adjust, "5 minutes -(on a clock?)- eguals 10bar of adjustment. If you use .22 and follow the simple one page directions you can get 60 shots at 25-26fpe with the same 10fps max spread, likely you can boost it a bit higher if that is your thing.
So a stock Marauder(.22) may get 30 shots at 25fpe with a 40fps spread and with the reg you would get 60 shots with a 10fps spread. The batch that came in last week is sold out (I believe) so you might want to email Vincent saying you want one from the next batch.
I hear they work very well in the AA rifles as well as the Monsoon. He can also get them for other airguns and is a pleasure to deal with.
John