Do you know if CloudFlare's apex CNAME support works coupled to Route53's health-check-based RRDNS? I know that AWS's own DNS reflects the health-check-based changes to the round-robin pools instantaneously, but I have no idea what sort of TTLs they emit.
Haven't used Route53 recently but those TTLs should be configurable by you. Obviously the lower the better to keep propagation time low for changes. CloudFlare has more info in the blog post [1] that says they do respect the TTLs that are given for records and then cache the value until expiration so I'm assuming they also send along the same TTL value of the original lookup.
There's no way to ensure the rest of the internet will handle it correctly though with all the proxies and DNS caches in the middle and low TTLs can also add latency to end-users who might have to constantly do a DNS lookup on new connections.
If you're using CloudFlare's full service (instead of just DNS), then it'll be seamless because their IPs don't change.
