I've been a Linux user for about 8 years. I first used Red Hat, but I prefer to use Novell's Open SuSE nowadays. The install was very easy, believe it or not, though a little hacking was needed to play Flash, MP3s, DVDs and some other things. There are guides all over the internet, so it was a breeze.
One big drawback to Linux is the lack of commercial titles, especially games. Office and other popular home computer jobs usually have a very good open source relpacement that is obviously cheaper and sometimes better than the commercial version (Amorok, the music player/manageent system compares very favorably to iTunes, maybe even better).
It can be tricky to use from an administration standpoint. You have to know how to install libraries because some packages will have dependencies, but many systems have admin tools like SuSE's YAST that makes maintenance a breeze.
From a user standpoint, the biggest complaint is usually "it doesn't behave like Windows", meaning the person is so used to the Windows way that they consider alternatives to be inferior, not just different. On the other hand, you won't be constantly annoyed with popup windows asking "are you sure you want to open that application, it connects to the internet and you might get a virus".
Another advantage with both Mac and Linux is the way it extends the life of your computer hardware. XP and Vista are bloated and inefficient requiring more and more hardware. Even an old 486 can be loaded with the latest version of Linux and still be useful as a print server or an IP masquerade box (though without a gui).
The nicest thing about, Linux aside from its superior efficiency, reliability, stability and user experience, is that the fact that the price (free) makes it easy for a person to take Linux for a test drive.
For the record, it is OSX that is Unix, BSD to be exact. The GUI is fantastic from what I can see, but as a long time Unix/Linux user, I can't see how you'd get by with less than 3 buttons on your mouse. (being able to cut and paste with only the mouse can save a bit of time and is very convenient.
Edit:
I stand corrected about the 1 button mouse. As for GUIs, I have agree that Aqua is years ahead of anything - it is really beautiful. A distant second for me would be KDE. It is pretty standard desktop stuff, but KDE apps are well integrated and look great. I haven't tried Beryl, but it looks great.
I personally hate Gnome, I think it is about as ugly as Windows. But to each their own.