Given the current state and direction of web development, I
would take some extra time and learn one (or two) of the popular
jQuery (jquery.com) is very popular and quite simple to
learn/use. It has been backed by Microsoft, who will add it to some
upcoming version of Visual Studio (or they already added it, I
Prototype (prototypejs.org) is sort of the granddaddy of Ajax
frameworks. It's widely used and supported. Several other
libraries, most notably, Script.aculo.us (script.aculo.us) are
built on top of Prototype, providing effects and other features not
found in Prototype. Prototype is, essentially, the base library in
Mootools (mootools.net) is growing in popularity and getting
some good press lately.
There are larger frameworks such as Dojo and Yahoo! UI. These
are very large Ajax libraries with a lot to them.
So, while CF's built-in Ajax, which I think is built on the
ExtJS library (that may be wrong), is good, you'll be better served
in the long run to know another Ajax library or two.