You haven't really established a comparable build yet but I concede would most likely have a better GPU at the same price point given that the iMac specs haven't recently been refreshed. The build not yet being fairly comparable negates the SSD example and reference to geekbench. I haven't heard to many gaming rigs that are quieter than an iMac.
Here's a build I quickly put together. The system I'm comparing to is the 27" imac w/ the 3.4GHz processor, 16GB RAM, and 1TB + 256GB HDD+SSD. Total comes to about $3200.
For a similar amount of money, I spec'd out a system with much higher end components. I deliberately left out speakers and webcam since I don't really care for them... but I could easily fit those in for the same price if I went with a lower-end case or monitor or motherboard or... anything really.
You're also not taking into account how a tower+monitor can save you money in the long run. You don't have to throw away the monitor with each new PC. A good monitor can easily last 8+ years - but an iMac forces you to buy a new one if you want a new PC. Also, if you want to upgrade the video card and leave everything else - you can.
Authorized Apple service providers are all over the place. At least 4 of them in the area I live. Always provide great service from my dealings with them. Doesn't need to be an Apple store. Whether or not the genius bar is horribly inefficient is a matter of opinion.
If having to wait 30-90 minutes beyond the time of your scheduled appointment is not horribly inefficient in your book, then you have loose standards. Over the years I've been to the genius bar at least 20 times at various stores for various issues - every time I had to wait well beyond my scheduled appointment even though I always made it a point to arrive at least 15 minutes early.
I personally don't know of any authorized Apple service providers near me. Nearest Apple store is about an hour away. It's easier for me to just mail my stuff in to Apple... but that's no more convenient than what any other reputable brand offers.
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Also, what OS would the u-built run?
Windows, nope.
https://forums.macrumors.com/posts/15125414/
Hackintosh isn't quite a Macintosh. Requires more tinkering and effort to keep running as well as the real thing.
A good Linux distro would be respectable. Some wouldn't even require much tinkering after initial setup. I would rather run Linux on that machine than make it a Hackintosh.
I would still prefer to have a Mac over that machine running Linux. Just my opinion.
Although I'm typing this on a mac... I personally prefer Windows. It's better for multitasking IMO (well Windows 7 is... Windows 8 is terrible for that...). I just find myself being a lot more productive in a Windows environment.
Even if that weren't the case - building a hackintosh isn't that difficult. In fact, there are some gigabyte motherboard which basically don't require any tinkering if you get a compatible video card. I think I would be willing to spend a little bit of time tinkering to get much higher end components than what an imac offers (or to save some money with similar caliber components).
Linux is worthless in a consumer environment.