I have been writing JS for 10-12 years now and jQuery (or other JS libraries/Frameworks) is a God send for any web developer. The bulk of my logic has gone client side in the last few years. Server side is become little more than a conduit to the database via JSON and XML and/or for serving up web services. It has gotten to the point where I use MVC client side and server side is just a service layer for the model.