I started using Clay this year, went with the Mothers and was very happy with the results on my XK and the wifes Jetta.
I will need to buy another kit as the clay was pretty close to toasted after doing her car and the Jeep, and then I dropped it on the ground. I had planned on sticking with the Mothers, even though I use Maguires for most appearance related products. I have also read that NAPA has a good clay kit for about the same price, not sure about the quality of the lube (detailing spray) with that set, I really like the results of the Mothers.
2007 XK 4.7 Sport (stock for now)
2005 TJ 4.0, intake and exhaust mods, 31's, 2" lift.