As mentioned above, usability can be improved very much with AJAX, but it might also be just too much. At Backbase we don't have a strong opinion on this: depending on the application and the audience, just choose an appropriate interaction design. We support both very flashy, but also very boring and functional sites (including desktop-like applications).
to see an example (use View Source and ten follow the b:link, b:include and b:buffer tags).
Our engine has to work around all kinds of browser quirks, so you don't have worry about them. The consequence is that it takes uw some time to support new browser versions, such as Opera and Safari. But we will support both of them, it's just a matter of time.
The Backbase client-side technology is server-agnostic. To speed up development, we started integrating with Java on the server first, but it now also works very well with .NET and PHP. Don't be fooled by the file extension though: our website actually has a full Java architecture, except for the index page which is PHP: it's easy to mix and match server languages with Backbase.
Regarding JSF: we believe we are not competing with JSF, but can offer a powerful extension on top of JSF. JSF is server-centric, and the Backbase engine can move more intelligence to the client, while synchronizing events between client and server. Looking at the discussion above, it seems that many of you prefer JSF: are there other Java GUI technologies that you like to have supported? How would you feel about declarative development, ie. defining the GUI in XML, like in XUL, XForms or XAML?
Looking forward to your input.