Edit: Updated to Beta 4 just fine, BTW
I love this board! It takes a bit of work to get going, but the GA-Z97MX Gaming 5 is a wonderful board for your own, bulkier version of the iMac. Indeed, I just taped mine to the back of my Cinema Display and I can't see the difference. From the front, at least.
It's not perfect, at least not yet. I have issues with kernel panics at boot (related to kernel cache and/or patching—I welcome any thoughts or suggestions). I'm not sure if I can fully utilize Apple's online services. And perhaps most importantly for the novice, Yosemite throws in some instability issues and graphical glitches (I didn't experience them in Mavericks). It's not the easiest or most welcoming build and will require patience. But if you can stomach those things it's a superb choice of quality and features. Let's just hope we get full support and stability when Yosemite is released, and that OS X will see official 9-series chipset support before Apple switches everything to ARM or some craziness.
Haswell Core i5 3.5 Ghz CPU
16 GB Corsair 1866 Mhz RAM
GeForce 760 MSI ITX Gaming GPU
Z97 chipset (As of writing, not used in any Mac but seems to work like a Z87)
ALC1150 audio with swappable op-amp
Atheros Killer E2201 (I think) gigabit networking
6 SATA III ports, a SATA Express port and I think an M.2 port as well
My UEFI BIOS Settings
Load optimized defaults (unless you really know what you're doing)
XMP #1 Enabled (enabling my 1866 RAM)
All boot options set to UEFI or UEFI Only
Optional: Enable ErP, ASPM and other features if desired, but only after you've successfully installed and booted (save yourself the headache )
None of what follows is my original work, so I'll be sure to credit the source and provide relevant links.
Obtain the latest version of Clover, the wonderful UEFI boot loader that makes the rest possible. Follow the directions in any Yosemite guide to prepare a USB drive for installation.
Install Clover to that drive, following the "For UEFI Motherboards" instructions. Use my config.plist.zip, or take care creating or modifying your own. Store Mieze's superb AtherosE2200Ethernet.kext, toleda's clever CloverALC script, rampagedev's SSDT and of course FakeSMC.kext from HWSensors on your drive. You'll recall the last from the Yosemite guide, no doubt! Build and boot your machine to UEFI/BIOS, and configure your settings (note mine above). Boot your USB drive to the OS X Installer. Refer to Clover's documentation on fixes and kernel boot options if you're having trouble.
Installation should run without issue. I recommend partitioning your drive with Disk Utility first, using GPT. After installation, feel free to reboot. Keeping your installer USB attached should allow you to boot your newly installed OS, but... ... You'll need to install Clover on the system drive, so be sure to do so and remember to include your FakeSMC and netwokring kexts for injection. toleda's script should take care of everything for you regarding audio, except he currently requires a manual patch for this particular CODEC. It isn't hard, and the necessary patch is in my config.plist.zip.
And you're done! Or at least you're as far as I am . Enjoy.!
I still get kernel panics that I cannot attribute to any particular source (likely audio—it was a pain to get working in Yosemite for me). Sometimes networking doesn't come up. Don't even ask me about Messages, "Convection" or whatever new Apple services are coming out. No guarantees that this build will be easy or worth your time, but I sure enjoyed getting it running. That said, a few considerations for troubleshooting:
I start by trying to Safe Boot (hit spacebar in Clover with proper OS selected). Does that work? If not, where are you getting stuck? Check your kernel boot options and other Clover settings, and make sure your kexts are present and in the appropriate directory on your boot drive.
Can't get audio to work? I often have to rebuild the kernel cache then reboot.
sudo touch /System/Library/Extensions
sudo kextcache -Boot -U /
Networking is pretty solid thanks to Mieze's kext. But I do sometimes have issues that a reboot or reloading of the kext seems to resolve. Go figure.