I like the build and the finish of the Surface Book, It doesn't feel flimsy at all, actually it looks and feel very much like a high quality product.
Performance-wise, it's very powerful, the discrete NVIDIA GPU does an incredible job at handling anything from games to graphic apps. The pen/touch is extremely responsive, i've played around with Graphiter and Sketchbook for Tablet, and found no lag whatsoever, with realistic pressure sensitivity.
The screen/tablet part detaches quickly and feels very sturdy when reattached. At a tablet, the integrated graphic takes over, and while you can't play games or use graphically intensive applications as you would with the discrete GPU, it's still a very comfortable and responsive device for note taking, web browsing, and other lightweight tasks that make sense for a tablet.