Although I've been tempted to use jQuery Mobile or Sencha Touch for a mobile application I've begun working on, I've decided to just start it from scratch. The main thing that's kept me away from these frameworks is speed. For example, many of the UI elements in jQMobile just don't seem very responsive (testing using my old HTC G1) - you select them and then there's a noticeable delay before any action. I don't if it's just me, but I find that irritating. The page transitions in jQMobile are also slow and choppy. Sencha Touch seems to be a little better in this regard, but it's still too bloated for my purposes.
I think these frameworks will get better with time (especially jQ Mobile),but for now and in my case they're just not worth the bloat in spite of all (or because of?) the great features they have. On a side note, not all frameworks are out of the picture. I am still considering using Backbone.js or Sammy.js to sort out all of the application logic.