Define "upgradeable".
If that means being able to swap out major system parts like motherboards and processors, than yes, Macs are quite limited in the hardware department. But that's just not the niche that Apple caters to. And for DIY enthuisiasts, Apple cannot be a viable option for that among several other reasons. But you can still add PCI expansion boards, add more memory, etc. Besides, by the time you're ready to make a major system upgrade such as a new CPU/motherboard, you're gonna need faster RAM and more demanding hardware components anyway. So I don't see a huge difference aside from the fact that Mac hardware admitedly demands a premium.
I was a DIY-er for over a decade. I used to hate Macs A LOT. I never thought that I could actually justify dropping over $2k for a desktop machine that didn't allow for an upgrade path outside of adding expansion cards, RAM, and maybe a larger hard disk. But that kind of thing doesn't seem to matter to me much anymore. What does matter to me now is that I can go home after a long day of fixing other people's PCs at work and just sit on a machine that does everything as gracefully as my PowerBook G4 does. I essentially bought into Apple for OS X, which is everything I've wanted an OS to be for years; but nobody could seem to get it right. Because its foundation is UNIX, there has been quite a healthy number of developers actually developing for Mac and thorougly enjoying the platform at the same time. They must be doing SOMETHING right, because they convinced me and several of my Windows-using colleagues to try something different. That really says a lot when you can build a reasonably satisfying PC for nearly half the price.
I use my Mac for video editing and screenwriting - that's essentially the brunt of my work, as I am a film student. Final Cut Pro is amazing - and it doesn't give me a lot of the same stability problems and annoying quirks that Premiere on the PC had in abundance. The secret behind Apple has always been a streamlined, elegant interface - and I will be the first to say that things like that are not for everyone.
My 2 cents...