Lately, I have had the opportunity to add some pretty funky usability behaviour on websites thanks to JQuery and some UI Plugins.
I have played around with sliders, treeviews, tablesorting, date pickers and ajax forms and they have all been a dream to implement. Typically, we would start estimating stories for one of these functions as a Medium, but then as soon as we found and used the plugin, we realised it was actually a Small.
Of course, not all plugins are easy to use, especially when looking at cross browser issues. My rule now is if it is not on the JQuery plugin list, then I won't even bother looking at it. I also note who has written it - you will notice that a large number of good plugins come from one or two authors - now, I trust any plugin that they write and will use it if there is one there. My other rule is to spend no more than 10 minutes trying to make it work. If after that time I still haven't got it working (on at least all browsers other than IE) then I look for another plugin, or I roll my own.
We have been loving working on the stories which require funky usability controls, so much so that I have introduced a new motto for my current project - jQuery is your friend.
All in all, however, you will find it a dream working with jQuery.