Well I'm running 20 and 18 at the moment.
Actually used it on some B roads with Mike (theman2k) and his v6 over the weekend. The car felt MUCH better on the bumpy roads with the damping right up. It's all down to what springs you're running. As the springs are fairly hard, the damping has to be set equally hard in order to 'balance' the cars handling.
I ran 10 and 8 for a while but found it quite crashy over bumps as the damping was far too soft for the spring rates I run. I am pretty sure I could wind it right up to max without the handling suffering on b roads, and as soon as you hit the smooth tarmac its's even better.