Gnome 3 was the first one I looked at, and by far the most revolutionary. Everything in the new Gnome Shell is set up to be doable with a couple of mouse swishes and one or two clicks. A swish to the upper left corner of the screen does three things -- does an Expose'-like scale of your windows so you can choose which window you want to activate by just clicking on it, sucks in a dock from the left side of the screen, and sucks in a screen list from the right side of the screen. You can grab a window and move it to the next screen in the screen list (there's always one blank screen at the end of the list -- sort of like the iPhone's applications screens), or you can click on the word "Applications" towards the upper left of your screen and see a list of applications in a format somewhat like an iPhone's application chooser. It is all easier to use than it sounds -- it really does make the whole thing swish swish swish point and click easy.
If you have applets running, like gDesklets, you can get to them by going to the bottom right corner of the screen. A sort of fuzzy menu bar then rises up from the bottom.
My general conclusion: Gnome 3 is currently incomplete -- it's barely configurable at all for example -- but it puts together the best ideas in UI's that have come down the pike over the past few years into an easy-to-use whole. My workflow falls out of the way Gnome 3 works naturally. If I want a screen for my browser windows, for example, swish to top left, swish to next screen on the right and click it, swish to left and select my browser on the dock, and voila, it pops open on the new screen and *another* blank screen is created for the *next* thing I want to do. Close all the windows on a screen, and it goes away, so I always have just the screens I need for my workflow -- no more, no less. I spent some time today doing software development with this system, and the usability compared to traditional Gnome is astounding.
Next up was Ubuntu's Unity. That, alas, turns out to be a disappointment. While Gnome 3 re-imagined the world to the point where some of the rumored Windows 8 functionality is going to be a clone of already-existing Gnome 3 functionality (like the iPhone-like application chooser), Unity simply attempts to clone Mac OS X without seeming too obvious about it. The problem is that simply moving the dock to the left side and modifying GTK+ to move application menus to the top like MacOS (except active and context sensitive, can't forget that!) isn't enough to make a quantum leap in functionality. Frankly, it ends up looking a bit of a mess. Gnome 3 re-imagined, Unity cloned, and like most clones, the clone isn't the equal of the original.
So: Does this mean the Linux desktop is usable now? Am I going to abandon my Macbook Pro and run a native Linux desktop again? Uhm... not hardly. I have a major investment in MacOS professional music software for which there is no Linux equivalent, and I still have significant difficulties viewing multimedia-based sites with Linux. Part of that is Microsoft and Apple's fault -- they release all these tools for "free" that produce (and view) multimedia content in their own proprietary formats like Quicktime and WMA, and don't release the viewing tools for Linux. I also can't read my corporate email using Linux -- neither of Evolution's Exchange plugins will handle proxied Exchange servers. That's sort of important too. Still, the fact that there is now a Linux UI which is actually innovative rather than a crude clone of other people's ideas is a sea change in an OS development process which all too often has ignored the desktop in favor of server optimizations. I don't know what happens next, but I suspect it'll be an improvement. Of course, given where the Linux UI started -- as an incoherent mess (inherited from MIT X11) -- that's sort of faint praise. So it goes.