Looks nice, but was sorely disappointed there was no self hosting option. I would be very interested if I could keep my data on my servers, but cannot move to this otherwise.
Even if it was possible for me, I still would not want that level of information out there with a third party. It increases the surface area for an attack, and puts the company at greater risk relying on an unknown level of security which there is no direct control over.
When it comes to my personal todo list, I'm ok with that. When we're talking about a project management tool that becomes a part of your team's culture, concerns mount quickly.