I've been using the Appbuilder for roughly a month now. I'm excited that I can work from OSX and Windows; however, the amount of bugs present in the simulator, especially when using "Developer tools", outweigh the benefit of actually using the platform to build applications. Overall, if I prototype by "right clicking" and editing live properties on the simulator, I can expect my device simulator to crash within 2-5 minutes of changes.
I'm not sure why this is acceptable with a production level app that users pay for. This platform's QA and dev quality would appear to suffer greatly from inner workings of Telerik's Appbuilder dev team. Much of this should have been tested more thoroughly, and users should not be charged for the constant instability of the application. I'm not able to be productive with constant simulator crashes.
This is an opportunity to open a world iOS dev to the world being executed poorly. I really want this to work well, but my impressions are that the Ionic framework's stability is much greater than what Telerik has built on top of it.