Yes, I ordered direct from them.
No, not really, it's the final rinse that matters. So wash the car normally, then just hook up the filter when you rinse the car for the last time.
If you order direct from RG, you can also order a little mesh filter to stick out the outlet side, so if you use it with a pressure washer, you wont risk pulling the resin through. Only a quid, so worth tacking on to the order if you like to rinse with the PW.
However I think most people would do a final rinse with just an open hose anyway, but as per a few posts back, I can now do a proper touchless wash when the car isn't too dirty....snowfoam as usual, then hook the pressure washer into the Raceglaze and just blast off the foam, and leave it - done.
Seriously, they are awesome bits of kit. I regret not buying it earlier. Drying is always the most tedious part of car washing for me, and the bit where I always feel there's the most risk of swirling or marring the paintwork. But I live in a hard water area, and I can now just rinse the car and leave it to dry, even in direct sunlight, and there's no water spotting at all. It's ace.