While I agree that knobs can be a pain in the butt when using a tablet, once you wrap your head around the way SM likes to work, it can be quite accurate.
The trick with my Motion Labs tablet, as is the case using ANY mouse with SM, is to click on the knob you want to adjust then draw the mouse or pen in a straight line away from the knob and then take the mouse in a circular motion the direction you need to go. The farther away from the knob you draw the mouse in a straight line, the bigger arc you will create "turning the knob" and the more fine the control of the knob becomes. Once you adjust your approach, it can be just as quick as anything else in SM.
Practice a little bit and you can get pretty great resolution on those knobs. 8)~
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I'm only assuming from all of the iPad posts on here, that things work the same way as they would normally when controlling via a VNC. I personally have not had the pleasure of iPadding it up at a gig just yet.