Jeff Hawkins cut a block of wood to fit the overall size of the envisioned product and printed out a simple user interface, like the one he envisioned. He taped the printout over the wooden block and used a wooden chopstick as the stylus. This was rough enough that it only took hours to create. He then carried it around in his pocket at work for months to determine the desirability of the value proposition in the real world.
When someone asked for a meeting or email, he would pull the wood block out of his pocket, tap on it with the chopstick and then put it away.
After several instances when he felt it would've been useful to have the real product, only then did he decide to go forward with the product development of the Palm Pilot.
Regardless of utility, this technique rules.