I've been using the free CloudFlare service for a few months now, it did speed things up as far as page load time and it definitely helped with server requests.
Also, I did activate the CloudFlare feature in W3 Total Cache as well. I've had decent success with it after a bit of tinkering and can say that I have made a marked improvement on page load time for this site here: kimoverholt.com.
I discovered the biggest factor in slowing the page load is the affiliate ads as well as any of the scripts that need to load.
There's still more I can do but I'm learning as I go with respect to the pluggins capabilities
"You will have to change your servers to theirs."
You're only changing your domain nameservers (DNS). You still keep your hosting provider, which means your site server IP address doesn't change at all.
"I've had passing contact with CloudFlare Support, but did not find them to be eager beavers. In fact I invited them to this discussion earlier today."
Sorry if it takes me a bit to get out to here from a request. We're experiencing decent contact volumes right now, and I'm in the process of helping train new folks to tackle that load.
"I'm currently using MaxCDN. Can cloud flare be used in conjunction/simultaneously with another CDN? "
There is no conflict with using other CDN services. You would want to make sure that any CNAMEs you have for your CDN are present in your CloudFlare DNS settings (settings->DNS settings).
"Always a bit wary when someone else wants to take over your DNS! "
This isn't a requirement if you activate via a hosting partner's cpanel integration. We will also have CNAME pointing available in about 4-5 weeks or so (target is late July).