I've just started to use it. It's pretty cool. I can manipulate my pages without clunky page reloads. I use it for real-time form validations and to increase the speed of some of my apps: that is, only load data when the user asks for it rather than loading it all. Will AJAX really take off? Who can tell. It depends on whether coming technologies provide support for it. But, for right now, I like it.
I think Ajax is here to stay -- at least until the whole HTTP/browser/web paradigm gets the heave ho (which I don't see happening anytime soon). There's a whole class of interactive web applications that just aren't possible without the ability to make backend requests to the server without the need for a full-page refresh.
So my answer to your question is yes. If you are serious about web development, Ajax is going to be a necessary tool in your bag of tricks. [ April 09, 2007: Message edited by: Bear Bibeault ]
Hi. Thanks for all of the quick replies. I was leaning toward the thought that it was worth my time to learn AJAX. Maybe it's just my programmer paranoia taking over that was giving me some second thoughts.