Unity 8 still won't be the default in Ubuntu 15.10 though. When Ubuntu 16.04 LTS arrives next April, there will likely be two "stable" system images to choose from: One with Unity 7 and the old X.org server, and one with Unity 8 and the new Mir server.
With a lot of work, Unity 8 could be ready to provide a solid convergence experience on an Ubuntu phone later this year. But average Ubuntu users won't be using Unity 8 until at least Ubuntu 16.04 a year from now. It's also unclear how many users will opt for the new Unity 8 desktop and how many will stick with the tried-and-true Unity 7 desktop.
Simply put, Unity 8 still needs a lot of work.
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