I did, largely because they take security seriously, but CloudFlare domains isn't a registrar: You can't set the nameservers to whatever you want. It's a CloudFlare lock-in service.
But you know what? I'm okay with that. I honestly can't think of a scenario where I'd want to use any other nameservers.
What about the scenerio where you are thrown off of cloudflare service? A CDN is more inclined to ban sites to limit their own risks from litigious IP owners, etc, irregardless of who would win an actual court case.
That could be an argument not to use their DNS service and their CDN service.
It shouldn't be an issue for the DNS service by itself.