If you're worried about lock-in, I'd first try Firebase along with our AngularJS[1] or Backbone[2] bindings. You don't have to change the way you write your app and can switch Firebase our for another backend easily. We recognize that we're only going to win a developer's trust by building a reliable service with the best tools and fair pricing -- so this is what we're going to do
I'd love to dispel any concerns, please feel free to email me anytime (james at firebase)