I've used the Zymol clay. I liked their waxes so much I thought I would try some of their other products. Like any brand some products were better than others, but IMO the waxes are really the jewel in the Zymol "crown."
Back to Zymol clay . . . as far as I could tell it was no more effective than any other clay it did last a long time and also was VERY tough to mold. As far as abrasiveness it seemed slightly stronger than an OTC clay like Meguiar's, Clay Magic, or Mother's.
I bought mine from Autosupermart dot com. I think the price has gone WAY up since I bought it years ago though. I also found that there is a local place near me that sells Zymol. I saw the clay in their display case, but I don't think the premium price gets you anything extra.