Gnome Shell 3.10
It took me a bit of time to get used to it, and in the early stages I went to KDE 4 for a while, only to come back to it later.
- Positive aspects: lots of space on the desktop, things don't get in the way, looks good,very good desktop overview (fast and well presented), a dock by default, great external monitor support (plug and play, remembers settings automatically), best OSD (volume) of all.
- Negative aspects: the notifications bar looks awkward and badly integrated (better with an extension), still unstable and big memory leaks (on Fedora 20, where the integration should be the best, it regularly crashes, starts with 300Mb and goes up to 1Gb in a couple of days), fallback-session completely useless as one can not customize it at all. But the killer for my work was inability to share the desktop with Webex, while XFCE could.
I gave it a long try especially in 2012, it has not changed much in 2014. My opinion of it fell when I tried it a very short time after months of Gnome Shell, and even more so after seeing the trouble my parents had with it, compared to Gnome 2.
- Positive aspects: desktop search (needs to be configured in order to scan only the relevant folders, used to be slow and resource intensive, not so much in 2014)
- Negative aspects: resource hog, awful start menu, too many shiny effects by default that only distract the user from his task, silly concepts like activities, every aspect of the desktop seems to require tweaking in non obvious ways for it to be more usable, looks ok but not great.
On Fedora, the default XFCE is very very basic, so much that I could hardly see a difference with one from 10 years ago. On Xubuntu, it's much much better. When I came to it from Gnome-Shell, I was surprised at how good was the "old" desktop paradigm for productivity. I also surprisingly found multiple desktops more natural to use than on Gnome Shell/KDE.
On Fedora the way to make it like Xubuntu is to install elementary icons, the whisker menu and choose the greybird/bluebird themes.
yum groups install "Xfce Desktop"
yum install xfce4-mixer.x86_64 xfce4-whiskermenu-plugin.x86_64 xfce4-cpugraph-plugin.x86_64 xfce4-mount-plugin.x86_64 xfce4-icon-theme.noarch google-droid* elementary-xfce-icon-theme.noarch xfce4-volumed.x86_64 pavucontrol.x86_64
- Positive aspects: fast and lean, great start menu.
- Negative aspects: external monitor support could be more automatic like Gnome-Shell, no nice overview of all windows, default installation can be a bit too bare, sometimes not sexy (volume applet is ugly, xubuntu provides the unity indicators in xfce as a remedy), primitive OSD.
Cinnamon, Unity, Conclusion
I gave a short try to cinnamon as well, in hopes that it was more stable than gnome shell. In short, it was not. It's certainly less of a memory hog, but I had some strange behavior with an additional phantom panel sometimes appearing at the bottom at the screen. And overall it looks a lot less polished.
Unity is more interesting, but it's too Ubuntu centric, I don't like the start button equivalent (slow, badly presented, don't care about HUD), the windows overview is not as useful as Gnome shell, the dock, something I usually like, is strangely annoying.
This is a very subjective review, my feeling is that in 2014, people should not waste their time with KDE or Cinnamon. Gnome shell could be worth a try if you don't care so much about memory leaks and slight instability but value a distraction free desktop. Otherwise go for XFCE or Unity on (X)ubuntu.