First off, its Mac, as in Macintosh, not MAC. MAC is an acronym for something else.
Yes, it is a closed platform, and yes the selection of applications is less than windows based platform. However, I digress, it has industry leading applications for professional environments. Adobe, Logic, and of course Final Cut Studio (which now offers audio rendering). Having a closed platform allows for less quantity of applications, but the ones that are available are of high quality. Thus, you don't have the copious amounts of incompatibilities and hardware/software conflicts you have with an open platform, ie x86.
Now, that Apple has entered the x86 platform there are more applications available to it with things like virtualization and cross over (which is a set of APIs that allow windows apps to run natively on OS X).
Also, go price out two PCs, a Dell versus a Mac Pro, and use the newest hardware (woodcrest) and you will see that the mac pro is actually around $300 to $400 cheaper. Not to mention look at their licensing costs. 5 User license for Tiger OS X is $199.00, Unlimited seat license for OS X Server, is $1,000. The money you save in mere software licenses is astonishing compared to other companies. Win2k3 server is around $4,000 for an enterprise license.
Both have their merit, and the low end macs tend to be more expensive compared to low end PC counterparts, but their high end machines are very comparable, to even custom build systems in price. Saying they are over priced is just not true anymore.
What OS X has done over Linux, is made a very intuitive UI with the solid background of Unix which makes it easy for non unix people to make the jump into a Unix OS.
Also, saying it is not compatible is also just an ignorant statement. With things like Parallels, Cross over, CUPS, LDAP support, GIMP Print, and even support for the windows AD environment (even though right now it sucks, but it can only get better) make the Mac platform a formidable opponent for market share. I think with the release of Leopard, and the features it offers to end users will make it even better.