I am finally—after months and months of planning—using EXT.
You can't even call it by its old name (YUI-EXT), because Jack ported it to both jQuery (yay!) and Prototype/Scriptaculous (meh...).
After I learned exactly what libraries and stylesheets one needs to include through testing (and this took awhile -- where the hell is this documented, anyway?), it was a piece of cake. The built-in Vista theme is going to work great in the coming months, too, as I'm slowly moved into a Vista programming environment.
I was worried that it might not work well in Domino's required HTML 4.01 DOCTYPE, but it turns out it works fine -- it's what the Documentation for EXT is made in!
(Special thanks go out to my boss, who allowed me to upload the entire 15MiB SDK to the Domino server in one big chunk -- there's no way to explain how easy that made everything).