Our team tried both. Originally we had a real JS guru on the team, and he wrote a ton of code against ExtJS. It was good stuff, but very difficult for anyone else to understand. He ended up leaving the company, and I inherited the front end code base. The project got put on hold for a while, and we decided to take a fresh start. I went with Jquery UI rather than ExtJS, because it just felt too much like you had to drink the kool-aid with ExtJS. Our team was more comfortable with a hybrid approach.<p>Every form we write is in bare bones partial HTML, and we use some juiced up JQuery UI Dialog code to display the forms as modal pop-ups. Additionally, we use the UI themes and their css classes even for non-JQuery Ui components, that way when we switch themes, all the elements change, not just the JQuery UI ones. There are only a couple of palatable themes available, but on the whole, it's been nice.