I've used jQuery's AJAX on a webforms app (kill me), but it worked fairly well after fighting with the webforms framework for a good while. I had links that would load aspx pages into a content div through jQuery's AJAX functions. One of the biggest issues was dealing with state without using session. I ended up using cookies that were used on the client and server. A major security issue, but the site was internal and not in the cloud so I didn't have to worry about that.
It navigation works great for the user and the speed is amazing, but it obviously isn't the best way to go (using webforms). I'm going to build a MVC app with the same navigation scheme. Anybody have any experience doing it a similar way? Any tips or advice?