I've gone for a hybrid approach where we user Sinatra as a base, ActiveRecord / Postgress etc to serve up page routes (slim templates) expose a REST API the web-app can use. In early development stuff like populating select options is done via helpers rendering into the slim template, but as we approach production this gets swapped out for an AJAX call to a REST API as we start to care more about page-load speeds and so forth.
Stuff that's easy to render out in Slim gets handled that way, and stuff (populating forms, receiving form POST data from jQuery.Validation's submitHandler etc, is all abviously AJAX)
Testing is an issue. Right now I'm stumped trying to pass JSON data to a Rack::Test POST test.
