I've worked with alot of different Javascript GUI Frameworks such as Bindows, ExtJS, Backbase, Javeline, jQuery UI and all of them can be really great in their own right as long as you understand the kind of webapplication you will be making when you start to rely more on Javascript for UI then plain ol' vanilla HTML.
If you really need a rich user experience with treeviews, collapsable panes, listviews, datagrids, comboboxes. Go with one of the Javascript UI frameworks but be aware that jQuery UI does not compare to bigger brothers such as Bindows and ExtJs.
GWT and Bindows both offer a client side language that can render all the javascript for you and might be a very good pick if Java is your developers language of choice.
If you only want little bits of ajax and some small dom manipulations revert to plain old HTML and spice it up with jQuery. Personally this always wins for me since you're more in control of styling, and all the bits that fly over the line. Javascript UI frameworks tend to be big and hard to style.