I tried on two machines, a 13" laptop and 40" combined dual-monitor desktop. Worked great on the laptop: I really liked it. On the desktop, though....
For one, mirror the menus is a big improvement compared to anything. They tray (notify area) was on both monitors, the focused app's menu stayed on that monitor (compare to multi-mon OSX where the app's menu shows on one central monitor regardless of which monitor app is on).
But the launcher sucks. It lives on a single monitor, and being on the horizontal edge, means I've got potentially 40" to slide the mouse before I get to the bar. Now, it's got a text-driven launcher app too, like Spotlight or Gnome Do or Windows' start menu. That always runs on the primary screen. And then the app launches on the primary screen. Whereas with Gnome Do, it summoned on my active screen and then launched the app on the screen its active screen.
Unity lost 10 years of advancements with multiple monitors. And that makes it a huge pain.