Here's my two bits - I just bought my first Apple computer, the imac 27" base model. Over the years (decades actually), I have never wanted to pay the Apple premium for hardware that could be assembled under Windows so much cheaper. As to Apple software, there wasn't a killer application that I had to have that was Apple specific .... although iphoto and imovie always looked so nice.
However, there are now two things that made me switch. First is the move towards iOS features. Touch gestures on the iPad are great and simple and I can do some of that on the imac as well.
The other thing I want to support or benefit from is the Mac App store. Although I don't use Aperture or Final Cut, I love how Apple lowered their prices significantly to the benefit of users (and themselves). I have not liked the inefficient pricing/business model of Microsoft and Adobe.
By the way, I've used Lightroom 3 on both platforms and haven't noticed any difference.
iMacs may seem overpriced, but you're getting both a pretty powerful computer and a very decent display. In case of 27-inch iMac, display alone accounts for more than half of the cost. Also all Apple displays, including built-in ones, are factory calibrated which is very good for photography. Of course, you can get a much more powerful PC for the same price, but it will require additional effort to get everything running as smooth as iMac will be right out of the box. And don't forget to count the price of Windows 7 which is not exactly cheap ($320 for the Ultimate edition). Just my opinion - and I own and use both Macs and PCs.