No doubt it is a useful service, even I use it for some sites. The thing I hate about it most is that they force all Tor users to enter an impossible to read captcha, making it impossible to access any websites that use it over Tor. Most of the time when I encounter a cloudflare-protected website over Tor I have to give up trying to access the website. The difficulty of the captcha seems to get harder as there are more attempts, so due to people using captcha bots over Tor the captcha's are literally not even characters from any language, there is no way you could enter it using a keyboard and if you ask for another captcha it just gets harder.

I also think that most webmasters that use cloudflare are unaware of the powers they are handing over to cloudflare and the enormous amount of trust they are putting in the service. Doesn't help that it's a freemium service either. If your website has any kind of user authentication then you should probably stay away from cloudflare and such services if you can.