Custom extenders, like custom controls, will have more volatility toward change on the server-side than on the client side. When developing components using the ASP.NET AJAX framework (not shown here) or the AJAXControlToolkit, there are some differences between the two approaches. But in the JavaScript client components, there is not.  It is the same approach, and it does not really change. In the future, as new additions come out from Microsoft, it still will not change as much as the server-code can, which is a good thing.

Developing these scripts by hand can be more difficult though, simply because of the sheer volume of code. I use CodeSmith Studio to generate my ASP.NET AJAX code, which I custom developed a script for my need. It was not overtly difficult to setup, and the benefits paid off immediately.

Once you learn the constructs to ASP.NET AJAX, it stays the same, so the major challenge is the initial learning curve. After that, the scripting portion is not that difficult at all; most of the difficulty comes from figuring out the complex processing logic and state management aspects.