We're not using ESI. We can't / don't want to use ESI includes in our framework due to the nature of our templating system.
Ideally we would use inline ESI but no one seems to want to share the know-how of that dark art wizardry. At least not without charging an arm and a leg for it, which to be honest is pretty uncool IMO.
So, we have a workaround that works pretty well for us. Yes the ajax calls add a slight load, but nothing really in the grand scheme of things.