Like I said before, it's the lack of tactile feedback that kills it for me. Having to *look* at the controls negates the purpose of having them. I did some tests, just for s***s and giggles. Ran through the same series of shots using my hardware (MC Color), Mouse+Wacom+Keyboard and the Tangent iPad app. Kept a rough stopwatch kinda thing going. Results? 'Bout what you'd expect. Or not, depending on what you expect. :P Basically the order I listed them above is the order they finished.
Again, what killed the iPad app for me was having to look down, then back up to the scopes, then over to the preview, then back down, and so on. Now, if someone were to build a similar app with scopes built in (Even crummy low-res ones like Color's) it could be kinda awesome. Still not a replacement for a hardware surface, though