I own an 2008 iMac, but just recently built a Hackintosh, running 10.7.3 for my girlfriend who couldn't afford a real Mac. I figured if everything doesn't work right, it's no big deal as it will more than likely be used to surf the web, play on facebook, organize photos, etc. What I found out is, the $400 Hackintosh I built runs circles around my $1400 iMac. Granted my iMac is getting old, but the Hackintosh is posting Geekbench scores around 7500, which is the equivalent of a high end 27" 2011 iMac for a quarter of the price (my 2008 iMac scored a 3500 by the way). I've (so far) have had no major problems. I also have working HDMI video out and USB3. It did take some setting up, but now that I know what I'm doing I feel I can confidently build a new one in under 2 hrs, that's building it and installing the software. One side note, the $400 does not include an internal HDD, as I used an old one I had laying around the house. Another thing you will have to add to the price is a monitor and a copy of OSX, if you don't have them. So, when you factor in the price of the monitor and HDD, you could easly double your price.
Also, the argument of a good warranty with the iMac is flawed, in that I get a better warranty on my individual parts. I believe my motherboard alone gets a three year warranty. Also, I can upgrade my computer to future proof it if I need to. It wouldn't take much (time or money) to upgrade the CPU or graphics card compared to buying a new Mac.
I'm going to play around a little more with my girlfriends Hackintosh, including running real software on it, like Adobe Creative Suites and see if I encounter any problems with it over the next few months. If everything works out, I think I'll be building one for myself here soon.