I think the disadvantage of it being harder to code is subjective. I personally find JavaScript based UI's to be both faster and easier to code. They also offer flexibility that was next to impossible in the old page post model. While I have developed web applications for years, I have always felt that it was disjointed with the old pattern of assemble a page then cram it down the pipe, then for ever event that happens on the client side, send it back to the server to see what to do. I have never liked the pattern and always felt that it made development far more complex than it had to be. It was fine when we where taking a form pushing it across the pipe and handing it off to some simple CGI routine, but we rapidly outgrew it. I prefer what we are doing now, where companies are building data and service platforms and UI's are rapidly assembled independently to take advantage of those platforms.