So...Unity. I gave it a serious try. I really did. I used it for several weeks, starting during the tail end of the Ubuntu 11.10 development cycle. I liked it at first. It was shiny and new (to me at least). Unfortunately, a number of relatively small but incredibly irritating annoyances began to nibble at my shins.
For one thing, it can't seem to get focus changes right. With the new global menu bar, this is a recipe for disaster. On a few occasions, I have ended up quitting the wrong application because I thought one window had focus when really a different one did. They both had a "Quit" entry in their "File" menus, though, so I didn't notice my mistake until the wrong application closed.
Probably more annoying, though, is that Unity and virtual desktops get along like Germany and France. Alt-Tab takes you back to the last window that had focus, regardless of which desktop is currently active. That makes switching between a few windows on desktop A and a few windows on desktop B an exercise in frustration.
Alt-Tab also handles multiple windows for one application in an incredibly obnoxious manner. Essentially, Alt-Tab switches at the level of applications. If you simply Alt-Tab to the application you want, you get the last window from that application that had focus. If you want a different window from that same application, you have to wait a bit. It takes a second or less, but then I want to do something with a different window, I don't want to wait. It's especially annoying if you keep switching back and forth between application A and application B, and application B has more than one window, and sometimes you want one window and sometimes you want the other.
For example, application A might be an IDE, and application B might be a web browser. You are writing code in the IDE and have some documentation for API X open one window of the web browser and documentation for API Y in another window of the web browser. You can't just put them in the same window, because you often have several tabs open for various pages in the documentation, and putting the two together in the same window would mean navigating a horrible mess of tabs. As long as you only work with one API at a time, this is fine. That is rarely the case. Thus, looking something up often takes an extra second. This gets very annoying very quickly.
I suppose these issues would not be as problematic for people who don't use virtual desktops or multiple application windows. I can see it being relatively painless on a netbook that is primarily used for web browsing. Of course, on that machine, you switch between tabs in the browser more often than between applications or windows, so Unity wouldn't have much opportunity to impress or annoy you...
I've always preferred KDE over GNOME, and Unity has completely failed to change my mind. The machine that ran Unity now runs KDE instead.
sudo aptitude purge unity sudo aptitude install kubuntu-desktop