There is a great deal of similarity between the current Macintosh OS X operating system and Linux. This is because they are both basically Unix. MAC OS X is based upon Darwin, an open source "flavor" of Unix, and Linux is an open source Unix clone. Thus, many of the MAC applications, such as the Apache web server, are the same open source ones that are commonly used on Linux. So underneath the covers, the machines are 90 percent identical in terms of the actual code they use to run.
The difference is that when you buy a MAC, you are paying extra for the proprietary Apple part of the software. You also get that nice Apple look and interface. However, Linux has become nearly as easy to use as the MAC, and you can now use all the standard MAC tricks such as dragging icons, files and applications to different parts of your desktop. See the FAQ page for info on how to make your Linux box look and feel like a MAC.
I anticipate that in the future these two operating systems will converge even more and become increasingly difficult to tell apart. My personal opinion is if you must have a MAC, get it! But, if you want something almost as good for a quarter of the price, go with a nice beefed up Linux box featuring a flat panel monitor and a DVD burner.
In addition to being able to run Unix applications directly on your MAC, you can also install Linux on it. This will make it easy to run the large number of high quality and free Linux applications that are currently available. I have installed Yellow Dog Linux on older G3 MACS. For newer MACS, I prefer Ubuntu, SUSE and Fedora Core 6. Note all MAC Linux Distributions use the yaboot program which enables you to multiboot and keep your current MAC OSX or OS9 install. If you have an old G3 MAC and do not want to pay for the OS X operating system, I will be happy to convert it to Yellow Dog Linux for you. I can also help you install Ubuntu, SUSE etc. on the newer MACS.