The first time since Apple introduced the split screen mode in OS X that it really comes in handy for me. Here I have Parallels running, on the left side with Windows 10 in the standard and on the right in HiDPI mode. This way, while programming, I can test how Task Till Dawn gets rendered in the particular display mode. Pretty neat!
Apart from that I find the split screen implementation rather quirky. I have to admit that Windows is one step ahead of OS X here. Apple only allows to have two windows side by side. Additionally, if one deactivates the system preference "Displays have separate spaces", the split screen mode is no longer usable. Completely illogical and inefficient – because if I am working with multiple monitors I want all monitors to synchronously switch spaces. But unfortunately, on OS X one can either use the split screen mode or the synchronous space switching. Not both at the same time.