Guess it depends on what you're going for! If you want a digital sketch specifically, then it seems worth it to try with a stylus and a doodling or sketching pr

Guess it depends on what you're going for! If you want a digital sketch specifically, then it seems worth it to try with a stylus and a doodling or sketching program. But Omnigraffle is not for drawing, it's for formalizing. Just like an outlining tool is not for thinking, it's for expressing what you've already clarified in your head. Computers require you to formalize -- make decisions, order, and choose form -- totally prematurely. I find pen & paper to be the best thinking tool there is because it's completely free-form. If you want the next best thing to paper, try that stylus hack and a doodling program, not a tool like Omnigraffle.