If you need to have the site display as secured I would, they also act as a proxy in that case which to some level can speed up page loads (as they would load from the server closest to the end user each time).
If you want to be using that you need to be aware that in the event of an IP change for your server/account (unlikely to happen any time soon, but it was an occurrence in the recent migrations), you would need to manually update the DNS section of CloudFlare with the new IP.
When you set up with cloudflare you will be provided with two name servers to use, and at the first setup they will probe the current name servers and duplicate what's on them, this can cause a problem in some cases as you may for instance not want to have mail.domain.com proxied if you normally use that in your email client, so you'd need to make sure to turn of the proxy option for any domain you need to use with a non web port.