I have used Vita-Rack, made by Hunter's Specialties, the last two years and will continue this year. I now have very well established sites, about one per 40 acres of land.
I made a mistake the first year and put some of the sites out in the CRP proper. One of the negatives is that the ground dried out so much in the direct sunlight that the deer struggled to "work" the soil. I moved my sites into relatively moist areas under tree canopy and that really helped.
FWIW - I attended a seminar last year sponsored by Antler King and the rep said if he could only choose between food plots and mineral sites, he would choose mineral sites. Both are preferrable, but the mineral sites are #1.