Yes it's true but the golden rule would be to build pages using php and add gadgets that use ajax on the pages. Indexing most gadgets would be useless for search engines.

I've got experience with 3 libraries. script.aculo.us (prototype based), sajax and jquery. I'm a little partial to jquery but the all suck equally. I'm not a great javascript guy so when I do ajax stuff I look for pre-built then I just massage it in place. This stuff is quite powerful though, and users really like the right ajax solution.

A couple of effects ... (If this type linking is not allowed just remove it, not trying to spam here).
1. drag & drop to save ordered lists to a database without submitting (menu in middle) (script.aculo.us)
2. sliding bars that update to db (trust factor, top right) (sajax)
3. (mac-ish) docked fisheye menu (top menu) (jquery)