We've found the GUI builder to be the most valuable feature for us.
To the best of my knowledge CN1 is better than some of the available mobile app development frameworks such as Xamarin and Nativescript because it gives almost 100% -- if not 100% -- write once run anywhere. Xamarin doesn't have this feature, you must change few things for some specific platforms. Compared with Nativescript, CN1 is better because of the GUI builder.
Furthermore, CN1 is cost efficient compare with Xamarin and Nativescript. Yes, Nativescript is an open source solution, but to test your app requires to be done either through the telerik platform or with a nativescript CLI simulator. Testing through the telerik platfrom is very easy but not free. Testing through a CLI simulator is free but difficult to setup.
I'm not saying Xamarin and Nativescript are not good framework for mobile app development, please don't misquote me. THEY ARE VERY GOOD. It depends on the ability and interest of the user.
All the aforementioned points are based on my own personal experience with the three frameworks (CN1, Xamarin, and Nativescript).