I already thought that AJAX was pretty darn easy in Rails. But the amazing Rails team has managed to make AJAX as close to dead-simple as is realistically possible. (I better be careful about such statements, next month they will probably prove me wrong!)
Cody Fauser has a nice writeup describing this new feature and showing how to use it.
Unlike conventional templates which are used to render the results of an action, these templates generate instructions on how to modify an already rendered page. This makes it easy to modify multiple elements on your page in one declarative Ajax response. Actions with these templates are called in the background with Ajax and make updates to the page where the request originated from.