Dario D. wrote a good post about Apple's current problem: selling Macs.
In it he makes a good case for why iMac pricing needs to be reduced and theorizes that most of the world is convinced to try a Mac, but that few people can actually justify one - when significantly cheaper PC options exist.
He cites a study showing the average PC is now $550 (while the average Mac is $1,543) and points out that you can build a triple-screen gaming/graphics PC that can handle any modern game, for $983 -- which is less expensive than the cheapest iMac (excluding the Mac Mini).
The age-old counter to the price argument is that you get what you pay for -- and total cost of ownership. Apple gave up the price game long ago, calling low-cost PCs "junk" and a "race to the bottom."
It simply boils down to some subjective factors, your OS preference (which is largely a function of experience) and your price sensitivity. Apple won't have a mass-market desktop computer in the short term, because it doesn't want to. However Apple's extremely popular iOS is dominant in the mobile space.
If Apple truly wanted to dominate the desktop (and I'm not convinced that it does) it could take a page out of the Chrome OS playbook and release a low-cost desktop and notebook running iOS that uses MobileMe for cloud storage.
Macs are definitely worth the additional cost, but then again, I'm biased.