Went with AdMo (http://www.admotours.com
), and had a great time riding at El Mirage OHV (<2 hrs north of LA) !
Russell (our guide) knew the area well, took us wherever we wanted to go, and the bikes were well tuned.
Was our first time in the desert, and it was fantastic ! Surprisingly diverse terrain; looser sand on the main trails, great traction off to the side (twisting through the sage brush), plenty of rocks of all sorts and sizes (with hills big and small everywhere), and a dry lake bed to rip about on !
Thumbs up to AdMo, Russell and El Mirage OHV !