Here is my take on iMac vs any windows computer. The iMac offers a sleek simple set up, literally one cable to be up and running. Yes there are better monitors, processors, video cards available. But you also get the headache of dealing with updating firmware etc with those parts. Since apple only builds 5 options (mac pro, macbook pro, macbook air, iMac, and mac mini) for its computers it is much easier to nail down any bugs that may come up after purchase. On a windows machine there are millions of combinations of builds and each piece could not play nice at any moment. Then you are stuck on a endless hunt for why something isn't working, been there done that and it stinks. With apple a problem comes up and it is addressed most the time before you even realize a problem, you just set auto update and it is a smooth set up.
Now even if any of that hasn't you convinced try selling your custom desktop for what a iMac will sell for. I would rather buy a used computer from a well known computer company than some guy who says he knows what he was doing when he built it. This is why you will see even rather old iMacs and MacBooks going for a good chunk of money. You get a quality build and you get to buy into the entire Mac eco system. Now if you run a iPhone, iPads etc having a mac computer makes it all that much more enjoyable. With airplay, Apple TV for wireless streaming your movies from your iMac without internet is pretty slick. Yes you could get a hdmi port on your windows and actually plug in your computer. But sorry I hate cables and hate turning my tv around anytime I want to watch a youtube video or show off new pics I got over the weekend and would much rather just turn on my airplay and be done. No cables, no trying to find some funky setting in windows 8 to get my pics on my tv.
So all in all its personal preference. Some people enjoy the challenge of trying to make something work. I prefer to use my time creating with my computer, not trying to figure out why my computer won't create. I don't want driver issues, I don't want to look at cables let alone plug them in. Yes there are faster components available but if the operating system is not faster then what is the point? If you just want to play games then great still think a PS4 or XBOX One is better, bigger online community and waaaaay more games.So if your claiming PC for gaming your kinda looking silly cause the boat left the harbor with console gaming and your stuck on land with the others who missed the boat all 6 of you.