It seems like if you are not building with responsive HTM5/CSS3 you may as well use a JS library for better UI. These are the two biggest names I have seen and it is difficult figuring out if either one is worth the time.
[0] - http://www.jqtouch.com/
See: http://code.jquery.com/mobile/1.3.0/jquery.mobile.structure-...
My experience with Sencha was completely different. It was very hard getting started as I had to relearn the Sencha/ExtJS way. I began to like the way each page/panel is "described" as JSON in a JS file. It's a completely different way of skinning a cat. Things like stores/models/views/controllers are baked into the framework. Support was absolutely abysmal. I think there were a couple of guys on their dev forums that actually knew their stuff. Quite a few "it works fine for me" type answers. Things blatantly left out of the docs which may be fine for normal users, but was extremely painful for a newbie.
In the end, I really liked the results from Sencha far more than anything I'd ever done in jQuery Mobile. I will say, my app in Sencha was extremely "templatey" which means it looked like a stock iPhone app... nothing custom at all.
The reality was that though it helped while prototyping, it required workarounds to do anything more complex than the token tabs+list-view/sub-view style app, and we ended up disabling all of jQuery Mobile's transitions and effects in order to get it to be performant (BIG difference there!). In the end, I'd say the costs outweighed the benefits in our case.
Next time, I'll just use one of the more general-purpose JS frameworks and custom style the UI myself.
With Jquery your app is an HTML page with your own markup which is then styled and functionality is added to your elements by the framework. I find it more designer-oriented compared to sencha.
With Sencha your app is usually just a single div and everything is done through JavaScript. I find it more programmer-oriented. It has been a couple of versions since I used sencha seriously but that is how it was at the time.