Debugging the open source AJAX toolkits is not as fancy. Dojo and Yahoo can log messages to a console, whereas Google has a complete environment that runs on your local development machine.

If you have a niche application, though, the proprietary toolkits can be your savior. IceSoft and Nexaweb (http://www.infoworld.com/2257) offer rich client tools that allow you embed a Web-savvy component in your client software. This inverts the AJAX paradigm and allows you to leverage all of your knowledge of HTML and JavaScript to build your client application. Kapow (http://www.infoworld.com/article/06/03/30/76786_14TCkapow_1.html) also offers a sophisticated screenscraper with an embedded JavaScript engine.

It's possible to do some of the same things with the various open source tools, but it will take more work -- a number of developers are using JavaScript interpreters to offer scripting, for example.