For the same money I would have spent on the SB, I got a MBP with better, larger screen, better trackpad, better CPU ans latest USB C ports.
I can't answer for anyone but myself, I purchased the SB before the new MBPs were out, so you need to exclude the advantages of USB-C.
For me there was a number of reasons why I opted for the SB over the MBP.
There is a need to use Windows in my day to day working environment
My view of Windows is that there seems to be more innovation occurring with Windows then OS X and I'm genuinely interested in what MS is doing.
There also seems to be more innovation on the hardware side, I like what MS was trying, the display is gorgeous, the battery is excellent, I'm very happy with the performance.
Keyboard and trackpad are excellent (trackpad is good for a PC, not as good as a Mac).
On the Mac side
Pre-2016 update: We consistently got updates that only made the laptop thinner, so basically, I was looking to spend over a grand for something that just failed to excite me on some levels. Yes its a tool, but I do want something looks cool and I enjoy using.
Post 2016 update: The new MBPs include a ginormous trackpad (15" model), smaller batter and poor battery performance, a keyboard that some people dislike because of noise, key travel and feel (other people like it and that's fine).
TouchBar is gimmicky, unlike a touch screen which doesn't take my focus away from my work, I need to look down and execute the action. keyboard shortcuts imo seem more efficient
Lack of SD cards, and magsafe. While many people tend to marginalize the loss of those ports, I do miss them and they impact my day to day usage.
I'm not down on Apple, though I've been critical of their recent actions, I may very well buy a 15" MBP in the near future, but regardless of what I choose, I expect to get value for my money. The Surface Book did just that when I decided to buy it last fall.