We've been using CloudFlare for two weeks now for an image serving site. It saves us about 1.5tb in traffic per day.
I still don't understand how they can offer this for free; I'd be happy to pay them a few hundred USD per month - it would still be a lot cheaper than AWS or any other cloud hoster out there.
You certainly won't be kicked off CloudFlare and I don't know what the parent is talking about. CloudFlare doesn't modify your pages to make money for itself. It makes money by people paying for premium services.
That section of the ToS begins: "Depending on the features you select, CloudFlare may modify the content of your site." The examples listed are things that you control, by default it doesn't modify the page.
They’re not free. Just enabling SSL costs 500$ for setup and then 100$/mo (compare to 20$/mo for Cloudflare). And they charge for bandwidth and requests.