I tried imgix for thumbnails in a web photo album. It was much slower (higher latency) and double the price per bandwidth of Amazon Cloudfront. In general, then, resizing all the images in advance and serving them from a traditional CDN offers a better experience at a better price. Only if that's impossible is imgix a good choice.

Being a premium CDN seems a bit like being a premium credit card processor -- as customers grow, they have more and more incentive to switch to a commodity alternative.