I am a paid Cloudflare customer. The service has been wonderful. As in
all services, there is a setup that is required and a learning curve.
One of the biggest reasons for me to implement it was to reduce server load. My forums keep growing and to reduce the need to keep upgrading, we tried Cloudflare.
It has reduced my bandwidth and server load immensely. For example, my bandwidth dropped from 45GB/month to 10GB/month. Server hits went from 3.5 million/month to 600 thousand/month.
Cloudflare is setup if the server does not respond timely, then your users will see an error message. This is an indication of a slow server versus a Cloudflare issue.
Who's the culprit- Cloudflare or hosting?
When I have seen the CloudFlare error screen for site is unavailable,
it has always been due to slow response on the webserver. (validated
by using a series of http responders hitting CloudFlare and the
webserver directly over a 60 day period.)
One of the keys is to make sure your hosting provider is using the CloudFlare extension and they have listed all the CloudFlare servers within your firewall settings. All your traffic will come from only a couple of IPs. If your server and firewalls are not setup to support this concept, they will trip DDOS or Flooding rules.