Form your own opinion by visiting their website for details. Cloudflare offers cloud based services for content including advertising and tracking analysis. The usage of their services is up to the client. I may use their service to speed up a personal website as the content will be served from fast servers.
CloudFlare provides a nice service working mostly well. Just do not use the "Always online" as that setting actually slows down your website.
For DNS CloudFlare is in top class, the best I have seen.
However, you should never give your credit card info to CloudFlare. They have a very bad habit of making what they call "Prorated Service Payment" charges at random intervals without permission. Usually it's only a few dollars each time but when they tried to steal $91.87 I got enough of them and moved to another provider for CDN.
Have found that they provide reverse proxy for sites that are bad netizens, such as xrchat.inf0 (xrumer support site, xrumer is known as a malicious forum spamming bot.)
Have also seen RFI attempts pulling data from servers behind their reverse proxy. They have been well notified of this, and defend themselves with a form letter, spammed to any forum which gives them negative press, rather than taking action and removing the offending parties from their service.
They also will cache and host these sites from cache, even if the original site is taken down. Sure they CLAIM they aren't a hosting company, but what do you call files served off a hard drive sent over the net, even if they are non-dynamic?
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CloudFlare is a free global CDN and DNS provider that can speed up and protect any site online