After watching their “movie”, I poked around the comparison page for the services they provide, and I ran across this:
Alert infected human visitors:
To stop the rise of threats online we have to tell people when their computers are infected. By joining CloudFlare, you help to educate infected human visitors that they need to take action.
Depending on how exactly this is done, it could cause some folks (who would already be infected with some sort of nasty) to jump to the (incorrect) conclusion that your site is infected, and at least some of those people will undoubtedly assume (also incorrectly) that your site is what infected them. This could, in extreme circumstances, lead to your site being unfairly “black-listed”, or even shut down. Now granted, the likelihood is low, but it still exists.