I'm looking forward to the evolution of the feature and hopefully it will get better over time and future releases of OS X.
It's definitely a big screen feature and works very well on my external 24" display. I don't want to change the resolution of my 15" internal display just to use it, although the 15" display works for most of the uses I have found so far. If I was only using a 13" display or smaller, I probably wouldn't want to use it anyway.
Apple are trying to move everyone away from sidebars, many of their Apps don't use them by default anymore and in OS X 10.11 when in full screen and split view, the sidebars are hidden, giving the user the full screen for the page content. Is good for the page content, more difficult if the user is needing to navigate around the app on a regular basis. If that's a good thing, we'll see.
One thing I don't like in Safari when set to split view, is the whole page is shrunk down including the text.