I have the Caldwell Rock but most of the time just use a pair of leather sandbags. I don't know if my experience with the Rock is normal or not but mine has a terrible wobble in the elevation adjustment. As you turn the big nut to raise or lower the rest, the rifle cradle sways side to side a LOT. Anytime you adjust elevation you have to adjust the rifle to get back on target. It's so bad that you don't have to be looking through the scope to see the wobble either. It's something you can live with but I was expecting better quality. Also, my Rock had another manufacturing problem in that one of the threaded holes that holes in the "feet" that you use to level the rest with had been drilled way oversize so that half the hole depth had no threads for the adjusting screw.
Obviously a Sinclair or one of the other true benchrest rests would not have these issues but they won't be nearly as cheap either. As I said earlier, most of the time I use an old 25lb lead shot bag filled with sand along with 2 good quality leather sandbags.