(segment.io co-founder here) - you integrate with one analytics API on the server-side - those events are sent to segment.io servers. you then pick your favorite analytics services (mixpanel, kissmetrics, etc) and we'll transform and route your server-side events there without you having to write and push new analytics code.
Does it keep intact original request host names and such, or do all requests appear to come from your servers? Also what about mixpanels user stream functionality - would this break stuff like that? I can see the value prop and usefulness of this, but it is just another fee on top of a fee, in a way.