If you want to add sophisticated client side functionality to a site, and are considering a javascript library like jQuery, dojo, scriptac….us, etc, remember:

• Powerful tooks often add significant overhead. It isn’t worth using a library to support a single pull-down menu.
• Try to find on-demand loading, where only the code required is downloaded.
• Consider themes offered by the toolkits before designing the site. They can greatly improve your design and will work better with the library functions.
• If you’re using AJAX, definitely use a library.
• Remember that you can limit the library’s impact by placing it only on those pages which truly require the functionality.
• Set up caching headers for the library code and CSS. They will probably never change (you shouldn’t change them).
• Try the libraries out before choosing one. Use more than one if you feel it is worthwhile.
• Use a CDN.
• Be prepared to invest a significant amount of time in learning curve.