What is your opinion about AJAX programming?

Do you have an opinion about AJAX (Asynchronous JavaScript and XML)? Write me a comment and share your experiences!

1. Did you use throbbers?
2. Did you use invisible iframes?
3. Did you use a URL fragment identifier?
4. What are the pros and cons?
5. What were your experiences as a user?
6. Which are the cooles AJAX sites you know?

Please feel free to write a comment!

Thanks

Bjoern