In the middle of a DDOS attack we found Incapsula. While expensive we had little choice. The setup was very fast, very simple and very painless. The user interface is hands-down super simple to use, yet there's a lot of horsepower available if one cares to dig deeper. The biggest surprise was how much faster our site operates and how much less bandwidth hits our server now due to the CDN. We signed up for DDOS protection, but we use the web application firewall and CDN on a daily basis.
The price. I don't know if it honestly costs what they charge to provide the service, but when a person is in the middle of a DDOS attack it feels a bit like they have you over a barrel. Having said that, if I recall correctly, their attitude at the time was, "Let's get you online again and then we'll figure out the money." So, while I dislike the price, their focus was very much on service.
Ask for pricing up front - it's possible to unwind the Incapsula service later on if it's too spendy, but it's not trivial. Also: If you think you don't need this service, you're wrong. You do.
We need 100% uptime (who doesn't?) Incapsula is a big part of our overall strategy to achieve that. We have increased our site security through the use of the Incapsula web application firewall and our site operates much faster now and uses far less bandwidth due to the CDN.