This is probably the best JS UI and framework I ever used! We've used it to develop a several projects in the last few years, including a major one and we love it!
The framework is full-featured, intuitive and easy to use, and it keeps getting better with each new version release. The amount of widgets and their features it has is truly astonishing! In addition to that, the Webix has some very advanced core features and helpers, as well as application development framework - Webix Jet.
It's apparent that the development team is really striving to create a great product. They are very active on forums and comments as well, helping the users and addressing the issues.
The documentation is rather complete for the framework of this magnitude, although at times it could be lacking or unclear. But even then, I've almost always found a solution through forums, or came up with my own workaround - the framework is very flexible and well-written.
Last, but not least - it's very reasonably priced and offers great value for the money.
In short - highly recommended!