Unity is the most popular desktop amongst users of Ubuntu, developed by Canonical, Ltd. It aspires to converge phone, tablet and desktop into a single platform with both a productive desktop and a lightweight phone. Unity uses a global menu, akin to OSX, and a launcher bar, which defaults to the left-side of the screen, with the added benefit of a fuzzy search for menu items. Pressing the 'super' key also provides access to a more full launcher window, including search for files and applications.
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Pro Easy access launcher
That is always accessible on the left side.
Pro Excellent HIDPI support
Pro Best keyboard bindings on desktop
Unity won a recent assessment of productivity for desktop users with the richest collection of keyboard shortcuts. Programs can be easily accessed by clicking on the Ubuntu symbol on the upper part of the launcher, or by pushing the "Windows" button found on most PCs. This opens up a box called the "dash" where users can search by typing in the name of desired programs to open them.
Pro Excellent touch screen compatibility
Unity provides a unified interface for mobile devices and pc.
Pro Appealing to both newbies and power users
Pro Basic tiling support
Basic windows tiling support for windows positioning and resizing with hot corners or keyboard shortcuts (Ctrl + Alt + Numpad), no additional extensions/plugins/software required.
Pro Does not get in the way
Pro Unobtrusive and fast
Content focused, it allow you to work and getting things done with no distractions
Pro Windows filtering
Can search windows with the keyboard by title/software.
Pro Works well with Multiple Monitors
The launcher can very easily be set to show on all screens or on a specific one. The Menu bar shows up on the "current" monitor as expected.
Pro Familiar to both Windows and Mac users
Pro HUD (search and use menus with keyboard)
One of the best and underappreciated features ever, HUD allows you to search and activate menus based on their name and what they do, instead of having to navigate them.
Pro Vertical panels work well with multiple monitors
Unity's vertical panels work perfectly with multiple monitors.
Pro Global menus results in more screen space
Screen estate is maximized thanks to the integration of the title, menu, and unity panel into a single global menu.
Pro Feels very complete
Unity is known to take a lot of system resources. Compiz also ends up frequently eating all your CPU.
Con Lacks customizeability of user interface
It's not possible to switch which side the dock is on, or make it not cover the entire side of the screen.
Con Files Manager lacks many features
The files manager based on gnome nautilus lacks too many features! With the removal of features such as compact view and split screen, newer versions of Nautilus File Manager are not as useful as they once were.
Canonical has ceased the development of the Unity desktop environment, along with the Mir display server. Future versions of Ubuntu will be shipping with Gnome 3. While branches of these are likely going to be maintained by the community, it's difficult to say how strongly these will last considering they were designed specifically for Ubuntu, they had never reached stable release, and more popular alternatives are already present (Wayland in the case of Mir).
Display issues with multiple monitors, particularly external output via laptop DVI/HDMI, and an odd file duplication bug when changing screensavers are the tip of the iceberg here and probably part of the reason Ubuntu have dropped Unity for the Artsy release.
Con Rubbish UX
The start button is right next to the windows button, so when you want to click the unity start button, you often end up closing an application, or clicking the browser back button because those buttons are just pixels away from the start button. Therefore, the UX/UI is very very poorly designed.
Con Does not allow choosing the side of minimize, maximize and close buttons
You can not change the positions of the minimize, maximize and/or close buttons.
Con Performance is bad, even when running on beefy hardware
It starts fast, but quickly slows down once you use the system heavily. Using faster computers doesn't help for some reason. This is running on a laptop designed for Ubuntu (System76 Oryx Pro) with i7 quad core, 32 gb ram, gtx 980m 8gb vram, ssd drive. So it's not a matter of weak or incompatible hardware or misconfigured installation. On this machine, sometimes it takes a couple of seconds to do a window spread animation (Super+w). Doing Alt+Tab is sometimes not as responsive as it should be.
Con Hard to run outside of Ubuntu
Con Very slow when running in a virtual machine
Especially clicking the start button.