I believe OS X to be the most polished and usable OS of the major named Operating Systems.
I used to be a hardcore Windows fan, but it has become my belief that the thing that used to be Windows' most valuable feature, the ability to install it on virtually any standard computer setup, has become its greatest hindrance. To make their operating system stable on the various platforms, Microsoft basically has to make a deal with the devil. XP and to a much greater extent Vista, feel almost bloated. OS X on the other hand fixes many of the ridiculous problems of its predecessors and changes its former hindrance (the fact only Apple computers could get OS X) into a major plus. With OS X, the user can know that things will always work in the same way because everything is built to the same specifications that the people designing the software use.
The first time I tried to install Linux, I ended up destroying my installation of Windows XP. At that point, I felt that if even technologically savvy people like myself had trouble with the installation, the operating system was doomed to industrial use only (ie servers, etc). I kept that attitude for a couple months, but the promise of some really cool technology bundled in Linux brought me back soon enough. This time, with everything backed up, I had no problems with my installation. This was only like two years, and at that time, the Linux installation software was about on par with XP's installation. Today, most installers far surpass the XP installer and even the Vista installer (I've never used the OS X installer, so I can't really compare).
Linux has come a long ways since I first ventured into this operating system. Its biggest problem is also its biggest strength: Linux isn't the name of one operating system, but the kernel on which "Linux" operating systems are built. Because the Linux kernel is open source, it can be freely (both in cost and in right) in a number of different devices we interact with on a daily basis. Many phones, servers, DVRs, GPS, etc use Linux as their core. The nature of what Linux really is makes it extremely successful.
Except as a Desktop OS. Now don't get me wrong, I fully understand that the kernel is extremely important to the function of an operating system, and while its operation is essential to the task of computing, it isn't the piece of the operating system that users most directly operate with. Users most often interact with the GUI interface of the operating system, and Linux distributions (or "distros") designed for desktop use must utilize their own form of GUI interface. The most popular distros use variations upon two major Desktop Environments (or DEs), the K Desktop Environment (more commonly KDE) and GNOME. I prefer distributions that use GNOME, as I find it to be the better of the two desktop environments, but both DEs lack many features that users of OS X and Windows take for granted.
Another major flaw with Linux is the fact that many popular programs just aren't offered for it (yet). If a software maker is willing to release their source to the public, it can typically be compiled on a Linux machine however this isn't for most users and I will confess that I often have troubles compiling software from source. This software manufactures with just one option: executable files... but Linux executables don't work like Windows and OS X software installation files. Instead of bundling everything, the maker typically only includes the software itself and not every file and library that will be needed to actually allow the user to operate the software. If you have an internet connection, most distros have nice programs that automatically download and install needed files from online repositories but if you don't, then you are forced to search all over for the needed file only to discover that you need another file to install that one... this is called dependency hell. It is especially problematic when a user needs to install special software to get their internet connection setup, but without the internet connection it can literally take hours to track down everything that is needed to install the software. Installations on Linux are further complicated because of the fact that different distros use different executable types. The most common seem to be .deb (for Debian based Linux OS) and .rpm. Because of this mess, many companies don't even bother making Linux versions of popular software and with a few exceptions, the open source replacement programs bundled with most Desktop Linux distros just don't cut it for most users.
Desktop Linux is always improving, and there is a lot of potential for this OS, but for most, it won't suffice for the types of activities people use their computers for, it just isn't ready.
I encourage people tired of Windows and unhappy with/scared of Linux to check out OS X. Most of the programs people need to use (like Office) have Mac versions, and Mac is bundled with loads of software so some people might not even need to install other programs. And for more technical users, OS X, like Linux, is a UNIX-like OS (OS X uses the open source Darwin kernel, built upon FreeBSD) meaning that many of the same cool technical features that are available in Linux are available on your Mac, including the ability to find ports for Linux applications.