I'm currently working on an offline web app based on ST2. So far the experience is positive. Since the framework is easy to add into a html5 appcache manifest, it's relatively easy to turn it into an offline app. The ui components are very high quality. I'm also really digging the mvc architecture with device profiles, which let you adjust your views for tablets or phones with common models and controllers.

Ofcourse it's not a native app, and you can tell. Fidelity is the downside of using ST2 instead of native. The upside is easier deployment (it's just a web app), and ios, android and blackberry support from a single codebase (windows phone is coming, but not ready yet).