While I'm glad that we finally have the ability to run multiple full screen applications next to each other, I must admit the way it's currently implemented leaves some things to be desired IMO... While joining apps is easy enough, splitting them up again is awkward. You have to either close or window one of them, the latter option throwing you back to the desktop each time. There is no apparent way to have it open new windows side-by-side automatically, such as in Safari. First Party apps are all over the place iTunes takes up the larger portion my 11" Air's screen, refusing to minimize any further. iBooks can only be placed in split view after you've opened a book. Also it maintains ridiculously wide text margins even when squashed down. Notes and Mail have a similar issue where their middle column remains visible even when squashed down, wasting precious space. Safari doesn't adjust font size to coup with shrinking window. Messages at least is thankfully fine tuned for split view. These are my thoughts on it anyway.