Well if this is an attack to get at one of DNSSimple's customers, running your own DNS would be a much easier target. Which is to say that if you were the target, you would already be hard dead by now rather than struggling as DNSSimple deploys defenses.
I agree though that it is a pretty simple service to run for a small domain.
If you're being targetted directly, then all of your services need to be DDOS proof, not just DNS. The more third parties you add, the more likely you are to be taken out by accident. If you have your own web server, you should dump Bind or PowerDNS on it and write a zone file. Problem solved.
I agree though that it is a pretty simple service to run for a small domain.