I've used it on a few small websites.

I think it's a great service if you don't want to spend money on a cdn.

I've used it a few times and it works well, it certainly didn't hurt. It's interesting to look at the stats they give and see how much bad traffic is being blocked. They definitely curbed the amount of comment spam on a test Wordpress blog I was using for awhile.

I wish I had more exact, measurable, information to provide but I used it only out of curiosity.

I tried them out for a bit, mostly because of their always-on claim but it didn't work for my sites for some reason so I switched to Amazon.

Their interface is nice, but I don't like the fact that you have to switch your DNS over to them and then re-enter data like MX records. With amazon I just point the parts of my domains I want to use to them and the rest remains with my primary DNS hosting.