What's the advantage of buying an iMac over building a Hackintosh? I'm not looking for a flame war or anything like that.

I am looking at (once I have some money, the hardest part) buying a iMac. Let's just assume one could build a fully working Hackintosh with one's current hardware.

What would be the advantage of buying a new 21" iMac running the same Mac OS X version vs. building a Hackintosh with better hardware?

The chances of me building a Hackintosh is low as I am, well, lazy. :) Just looking for reasons to buy a new iMac.
If your intention of buying a iMac/Hackintosh is to use it for a long term, then go for the iMac (or the Mac Mini/Macbook/Macbook pro with an external monitor). If it is for just playing around, experimenting or learning OSX internals(as @shpokas
 mentioned), or using for a relatively short term, the hackintosh is just fine.

If you think the iMac is too expensive for you, then hackintosh is definitely not the way to go, IMHO, I do not see any ROI in the hackintosh.