I have sub tanks on my YZ (YoT prototypes that my suspension guy got his hands on), and love them. I do a mix of track and off-road/woods, and they make a definitely noticeable difference in the plushness of the ride. Last weekend, I really noticed the difference in the slick stuff, and over a few downed limbs/branches. Like the poster above said, though....if you don't have the right springs for your weight, you are wasting your money if you put on sub tanks. The whole system has to work together and be set up together, it all goes hand in hand. It's extra money, but well worth it in the end, I think.
FWIW, I run mine dead in the middle, 4 turns out. I'm 250 lbs., and I've taken some pretty good landings from 15' +, with no bottoming.