Not clunky at all
I am building big applications with PHP (CodeIgniter) and Javascript (Ext JS) for my customers, and I can tell you that they don't feel so much difference with a desktop application. It simply feels like Windows with almost the same response. Concerning the offline with HTML5 offline caching, I would not use this as a selling feature to my customers. It is only realistic if your app and data can stay offline, without inflicting any consistency to the main system. And @fluciani, if you are working with XP, which is not supported by Microsoft anymore (like Vista SP1), then you should not be surprised that Microsoft will not wait for you. You are right that IE is a drama to test, but if you are a serious developer, then you make sure that your own tools are matching with the current standings in software. Don't adept to your most ancient customer all the way.
By the way, my Ext JS applications run without a flaw also in IE. It depends on the quality of the framework you use, and Ext JS is just super in common browser support.
