Bloated - yes a bit. I tried doing a custom build removing classes I didn't need manually, I could go down about 220KB and still have my app run. Similar things can be done to the css. It would be great if their jsbuilder could also pick and reject used/unused sencha classes to produce optimal build.
Choice of sencha touch also depends on where the majority of your user base lives. If I've to deliver something to India, 300KB in a webapp would be impossible given the great connectivity over phones here.
Poorly documented - well not exactly, in fact they've made a great effort to produce useful guides. API docs let your own add comments within the docs too.
Themeing - delightful, I get great control over how my app looks. I've built a nice abstraction atop their scss code that enables me to produce new themes for my app with 10-15 lines of code and deliver it to new customers. Thx to scss/compass.
Frequency of Pain in the Ass while working with it for 2+ months - very less compared to most other frameworks.
Amount of code you write over about the 300KB - quite less, unless your UI is completely completely different (mine is).
Integrating with your backend's APIs - smooth as silk, thanks to the awesome data store and model classes. They've been there since 1.1. They've been there since pre-backbone era.
Now they've got scaffolding tools to produce skeletons too, in case you were confused about the MVC approach which was unclear for quite some time. Studying examples helps a lot. My app has become too heavy - 20+ views, 15+ custom UI components, 8+ stores, 20+ controllers. Their MVC & the new class system results in a very maintainable code. Though I'm in the phase of re-architecturing my app and implement a view cache as seen in kitchen sink. I also feel putting in many sencha classes is heavy, I plan to reduce the views and controllers as much as possible. Our initial UI was done by a graphic designer in photoshop, it didn't care much about sencha touch's existing components. In turn trying to replicate the photoshop work as much as possible, has also resulted in some slow performance (translucent backdrops, shadows)
I'm heading on for a re-write with performance in mind. Been using sencha touch 2 right from alphas in Nov/Dec AFAIR. It's gone through some really sweet changes from 1.1. Kudos to sencha team for knowing what exactly their user-base needs.
Don't waste your time: Grab iScroll and roll your own mobile webapp.