Using touchscreens for controlling other software (using TouchOSC, for example) one thing I always had my problems with was a missing tactile feedback. Using a hardware Midi-controller, for example, you don’t really have to look at it that much. many things just work because your fingers will find their way over the buttons to the correct knob.
This might not be the most sensible thing to do with a smartphone but… It helps. Once you find the correct slot your finger is automagically guided.
There are so many reasons why this is not the most sensible solution of all times but with the correct layout, proper spacing and some …
It just feels right. In the true sense of the word. Try it.
And you can take a marker to write on your smartphone. I like writing stuff on things.