before you spend $$, get a large sock and knee-high pantyhose, and some airsoft bb's. put the pantyhose in the sock and fill the hose with bb's and tie off the hose and the sock. works pretty good as a rear bag and you can 'squish' it with your non-dominant hand to raise and lower the rear of the rifle as needed.
that said, i just bought the tab gear small and large rear bags to try but have been using the sock for years.
Myself, for heavy range rifles a bipod and small rear tab bag are what I use. For a light sporter rifle I use a front sand bag instead of a bipod and then my normal rear bag. I have pondered getting one of these though just to ponder. I bought a caldwell rock (lesser version than you linked) and returned it the same day.