I was skeptical about imgix, but I recently plugged it into a CMS for a startup I was helping out as a simple image editor + transcoding service. It did pretty much all the work: all I did was add a few buttons to alter the bri/con/vib parameters, and hook the crop up to simple jQuery cropper plugin.

We're also using it as a way to upload jpeg2000 files from iOS devices to save on bandwidth, which will then appear properly on Android and desktop.

I'd say it saved me a fair bit of infrastructure work.

EDIT: They had an outage the other day where new images weren't being cached that was fixed within a few hours (that sucked), but were awesome enough to provide a root cause for me.