Some early thoughts on split screen...
So I've been playing with El Capitan all day and I have to say, split screen is a bit unpolished right now...(duh right? Beta and all)
I find that the apps you select are very particular about how they want to be resized, iMessage for instance kept wanting to either be resized too small to use or would not wrap properly and you couldn't really see the messages part (I do like the minimizing of the contact info though to the pictures you have in your contact info, that is helpful).
The Photos app was very wonky...kept shutting down or freezing to the point I had to force quit when used in split screen.
Email seemed to work fairly well, but it did have some text wrapping issues of it's own.
I'm looking forward to seeing the progression of this feature, because it COULD be very useful, but it needs a lot of consistency improvement and just overall polish to really feel natural in OS X.