They are only sort of a CDN; in addition to DNS, they specialize in site optimization via content transformation (a la mod_pagespeed in the cloud) and "DDoS protection" (which is pretty much them replacing your website for new users with some JavaScript that tries to determine if you are a legitimate client).

They don't promise to cache much, and they in fact don't: even on very simple single-page static information websites, such as evasion.com, they have an abysmally poor 81% cache hit ratio. They don't help at all with dynamic content due to having poorly located nodes and lots of heavyweight code running in their proxy. Their lack of many nodes in good positions (compared to something like CDNetworks or Akamai, they are one or two orders of magnitude smaller) also means they can't provide very good latency even for the times when they actually happen to have something in cache.

(Note: if someone is now going to say CDNs don't generally do well with dynamic content, they are wrong: normal CDNs actually improve the performance of dynamic content incredibly by maintaining large-window pre-connected HTTP sessions to customer origin servers, often over private networks that already provide better bandwidth: you can easily see 2x latency improvements with a normal CDN even for fully dynamic content).

So, they really shouldn't be compared with a "CDN": they have an interesting service that actually provides something valuable for many key use cases (4chan comes to my mind: in essence, something that is actually likely to experience a true DDoS attack sufficiently often and with sufficiently little provocation that it makes sense to add an external system to your infrastructure), but if you need a "CDN" there are many more reasonable alternatives that don't have as many moving parts and are thereby going to break much less often (and, if they actually do, should break only in localized regions).