I was already on Linode and I'm only serving a few hundred GB of static files per day (with the Linodes I have this is well within my free quota).
In my instance (forums with current discussions) most static file requests are for image attachments in the very latest discussions, the hot topics. So Varnish fits this scenario really well. I didn't need a long-term storage of images in the CDN, I just needed to store the most recently requested items in the CDN.
Linodes are cheap, I was already using them in a distributed fashion to reduce SSL roundtrips, and introducing Varnish was a small configuration change.
I have tried a few other providers (most recently CloudFlare). But I was generally not happy with them, usually due to a lack of visibility.
I proxy http:// images within the user generated content over https:// when the sites are accessed over https:// . And occasionally I found that images would not load when I used a CDN provider for that. But never had enough data and transparency with the CDN to know why. Users notice this stuff though, so I'd have isolated users complaining of images not loading and no way to debug or reproduce it.
So I found that as my scenario made Varnish a good fit, and the bandwidth was within my allowance, and it was easy to do... well, I just did it.
I still experiment with CDNs every now and then, but largely I get more reliability and transparency from my own solution. I've also found this to be cost effective, though I would be OK with paying a premium if I found the reliability and transparency rivalled my home-rolled solution.
