Currently, the biggest asset of the style builder is the colorization - for really quick modifications. The fine-tuning is by no means complete (and I feel ashamed of some of the interactions which are unintuitive).
The tool will never replace a good hand-written stylesheet - its intent is to ease the burden and make skinning more accessible to everyone.
The grid skinning is only for its most simple form - we'll add more control states
soon. If you have other feedback, I'd love to hear it.
Regarding the tutorial/documentation - there isn't any beyond the videos. The problem is caused by the unintuitiveness of the tool - I'll strive to make it in such a way that it doesn't need documentation.
the Telerik team