I've been turned off of all these mobile frameworks. Tried many, but no one has "felt right". Some got completely different design patterns (Sencha Touch), some are almost dead (jQTouch), some are large, ugly and bloated (jQuery Mobile), use non-standard stuff (Jo framework), etc. Sencha is really close to emulating a native feel, if that's what you're looking for, and jQTouch is really easy to get started with. Pros and cons.

HP's Enyo framework seems really interesting though, as does Mulberry. But sometimes the best framework is no framework at all - Backbone + Zepto can take you a long way (but lacks fancy mobile UI widgets). It all depends on the project.