Definitely, AJAX is hot right now. Well there are many advantages using AJAX. I have stated few below.

1. Open Standards Based
2. Best Usability and User Experience
3. Cross Browser and Cross Platform
4. Benefits of Regular Web Applications
5. Low Incremental Cost
6. Plays Nicely with Flex and Flash
7. Adoption Is Strong with Industry Leaders
8. Web 2.0
9. Agnostic Server Technology
10. XAML, XUL, XForms

The most important advantage of AJAX is that, it is based on open standards supported by many browsers and platforms.