I have very mixed feelings about this. Yes, on the one hand this is great news because a lot of websites who otherwise never would have bothered with SSL can now be protected from snooping or traffic manipulation on your local (possibly very insecure: your neighborhood Starbucks' wifi) network.
On the other hand, this completely destroys the premise of HTTPS that you have an encrypted connection to the website you are visiting. If this catches on big time, seeing the padlock will only tell you that you have an encrypted connection to Cloudflare's network, and no way of knowing if the traffic is still encrypted beyond that, or that it's flowing in plaintext between Cloudflare and the actual target server. Worse, you will have absolutely no way of knowing if the content you're seeing is what the target server originally sent, or that it has been manipulated (or wiretapped) by Cloudflare itself or any of the other hops while en route.
If you are going to use this, just keep in mind that you're giving Cloudflare - a US company subject to the Patriot Act and the whole shebang of 3-letter agencies - the ability to collect, intercept, store, and manipulate every single byte of traffic sent between your users and your servers.