However, with the Surface Pro 4 (and all previous iterations), it seems impossible to actually use on your lap. Granted, I haven't had an opportunity to try it, but the kickstand and attachable keyboard seem like they only work on an actual desk. I very rarely use my current laptop, a MacBook Air, on an actual desk.
The Surface Book easily solves that problem, but I've seen reports that the battery life is only about ~3 hours with the screen detached as a tablet. That's low enough to make me question the purchase. I've had to wait at an airport for over 3 hours before, and that means that my tablet would have went from 100% to 0% battery before even getting on the plane. Granted, the keyboard base has more batteries built into it, so the device would not be unusable in this scenario, but it would not be usable in its ideal form factor.
(This article, for what it's worth, claims 4 hours for the tablet and 8 hours for the bottom -- slightly more generous numbers.)
Edit/update: I forgot when writing this that the Surface Book's screen can be attached to the base backwards with the base folded back, thus getting the full ~12 hour battery life (plus the discrete GPU) at the cost of some extra bulk. Depending on just how bulky it is, that just might solve the problem for me.