I used to run clip-ons on a MTB with slicks when I lived in PHX. 17 mile one way flat-as-a-board commute from 35th and Bell to Sun City West, they worked quite well. Good for about 2 mph advantage.

Back then there were much fewer shopping centers and traffic lights along Bell road, however. I doubt I'd run them on that commute if I were there today. Back then I only had to brake / shift gears once a mile at the most. Of course, the whole shifting thing doesn't bother me much as I still have to reach for downtube shifters on my main ride...