I use one. It's fun. :-)
I use it mostly when recording or setting up a mix.
I am a fairly precise mixer when it comes to final mixdowns and I use envelopes a lot. Touch is not the greatest for modifying envelopes.
When adjusting effects, they can jump around. If you are the type that often holds shift to get values to change very slowly, touch isn't going to be much fun.
When I hook up a second monitor, it gets finger prints. The whole thing about having one touch and one not touch I think is more b/c people want to keep using their existing monitor. You certainly can, but I have resisted buying another monitor that isn't touch. I just bring in a TV from somewhere in the house when I want 2 screens. I plan on another touch screen, but I haven't purchased yet.
You really don't need the most expensive touch screen to work with Sonar. I got one that was like $700! Totally unnecessary. At the time, it talked about swipes from the sides and stuff but, in Sonar, none of that matters. Any screen will do.
I totally recommend it if you have cash and want to get a sense of it. I'd never by another controller. I'd only buy another screen. You can use it as a keyboard! How cool.