What's the advantage of buying an iMac over building a Hackintosh?
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.