We use Thumbor at Yipit, and we are very happy with it. One thing that was not mentioned was extensibility... With thumbor it's easy to create new plugins and filters to extend your installation set of features.

We had detailed how we scaled thumbor at Yipit last year: http://tech.yipit.com/2013/01/03/how-yipit-scales-thumbnaili... The blog post doesn't mention S3, but we have a storage plugin that reads from and writes to S3.