It has its uses, but (like every other "new" technology we've had on the web) the vast majority of what it is being used for now is abuse and totally inappropriate.
Google Maps is a great example of a website that makes fantastic use of Ajax to deliver a rich user experience that you just couldn't achieve without it. On the other hand, far too many people are using Ajax for tasks that can and should be achieved with good ol' plain and simple HTML, and there's no reason or excuse for doing that.
Ajax. Just say no.