I've been using El Cap since the first beta (it's been surprisingly stable), and the one thing I think is a bit too weak of an implementation is split screen. It reminds me of when Apple first implemented fullscreen and it didn't support multiple monitors. My issue is this: it's as common a use case for split screen to want horizontally split windows as vertically split windows. But Apple have only implemented vertically split windows. Is this just for consistency with the iPad implementation? Will they extend it later? As a developer, I'd like my editor taking up the top 2/3 of the screen, and then a terminal taking up the bottom 1/3. With the introduction of System Integrity Protection in 10.11 it is no longer possible to inject code into running applications, which means there will be no SIMBL extensions to the split screen behaviour extending it. It also means extensions to Spotlight (Flashlight) will no longer work. I'm not sure how I feel about these changes. It makes sense to ensure the integrity of system files, but the whole thing feels anti-UNIX (root user is no longer the supreme authority), and very Microsofty to me.