We've been building up on Sencha Touch 2 since the first PR. It was hard to convince devs on team to make this choice. It takes some time to get into Sencha way, being familiar with ExtJS helps. Some benefits -

* a faster development process once you're clear on the concepts

* along with conventions, a lot of configuration is possible

* the new class system makes it easy to build custom UI components and write maintainable code

* the new class system combined with jsbuilder/sdk tools helps you produce minimal js.

* many well done examples

* ComponentView and DataViews are kickass. saves many lines of code that you might have written for updating views otherwise

* many types of eventful data stores, and they talk REST out of the box!

* great performance on iOS in general

* our customers needed a lot of theming - we build our theming system on top of sencha's sass work. rolling out new looks = 10 lines of change for us :)

* free :)