I built mine 4 months ago. I like it quite a bit. I still have yet to use it on a full session because I am still making the jump from PC to Mac and currently trying to get all the plugins I need before I use it fully. However, FWIW, I like it. I know Thedriller has had one and he helped me when I was debating building one.
I don't know how fast your imac is, so I can't comment on the performance increase, however, it seems like to me that my laptop with windows 7 and reaper is still smoother and quicker and easier. Maybe it's because I have always been a PC guy. We'll see in the near future.
technically speaking, follow Lifehacker's guide. Decently fast computer and no hacking required. Pretty much a standard installation of Snow Leopard.
http://lifehacker.com/321913/build-a-hackintosh-mac-for-under-800 That link is the original, and since then, there has been 2 or 3 updates to the installation procedure. As of right now, you have to make a start up disk on a thumb drive from your existing mac and add a zip drive with the prepackaged kernels ready to go, so no hacking required. Just get the exact Motherboard and video card that was used, and you're good to go. I opted for a slower processor (which I really regret right now) and less RAM, but ended up being 4 more gigs a week ago because it seemed too slow. Don't skimp on parts... But that's my opinion.
I'm running a dual monitor setup just fine, fyi if you are worried about that. Pretty much everything works great. My profire 610 is being a fucking bitch right now and I have to figure that out, but other than that, you should be good to go. Any questions, just PM me or maybe thedriller would like to chime in too. He was incredibly helpful when I asked him a few questions.
ps. It isn't "illegal". All building a hackintosh does is breach a contract. So, I suppose it is illegal, but if you go through and purchase a retail copy of snow leopard, that is as legal as you can do it.
