As some people seem to experience issues here with Mountain Lion, let me help thanks to my own experience. I bought this board on eBay last month, at a bargain price of 49€. the Bios was F13, the last non-UEFI, and I decided to give it a try first, before considering UEFI.
I first tried to install from the 10.8.2 build I'd just bought from the Mac App Store, but the installation failed several times, so I used a 10.8 build with Unibeast.
At last, it worked flawlessly, using internal HD3000 (my GTX6x00 wouldn't work out of the box, the screen went black).
After bootup, ethernet was working "OOB" and I run all available updates WITHOUT RESTARTING.
Then I opened Multibeast. FakeSMC plugins were bugged at the time (Multibeast 5.1.0), but the problem has been addressed since, so if you're an iStat Menu user like me, you won't crash like I did.
Settings:
• UserDSDT with corresponding DSDT on the desktop.
• Drivers>Audio>RealtekALCXXX>WithDSDT>ALC889
• Disk>3rd Party SATA (no IOAHCIblockstorageinjector, and it comes with a nice surprise: hotswap with a right click and no orange drive!)
• Misc>Fake SMC (+motherboardplugins, if you want sensors and all kind of monitoring, + application, if you don't want to pay for iStat Menu, a great app)
• Misc>USB3 3rd Party (installed but not tested)
• Network> Lnx2Mac (Ethernet won't work anymore on its own, once Multibeast has run)
• SSDT Options>Sandy Bridge Core i5 overclocked for my 2500K @ 4,19GHz (OSX 10.8 will crash over 4,2GHz, I checked)
System Definitions: I didn't choose anything, but it set itself to Mac Pro 3.1.
If you want to overclock and still save power, Mac Pro 3.1 is your best shot, since it'll set to using many different speeds to match you CPU activity.
About Trim:
If I'm not mistaken, Multibeast's trim patch didn't work for me, I used the Trim Enabler app from Oskar Groth instead.
About graphics:
• HD3000: don't touch anything
• GTX670/680:
A. Graphics>GTX 6x0
B. Customization>Boot Options>GraphicsEnabler=NO (prevent the black screen issue)
About other Intel graphic or 3rd party boards: I don't know, ask someone else! I'm not a true poweruser, I'm just using power to boot hackintoshes since 2008.
With these settings, everything is working just fine: perfect sound (Hifi user inside!), network (a bit slow on startup), shutdown, restart and sleep (from the case power-on button as well), my optical drive wakes properly (it didn't with Lion) and SSDT rules—proof from voltages drawn by the CPU, from 0,89V up to 1,25 with no OC, 0,96V/1,35V @ 4,19GHz on my 2500K.
I can hotswap any disk, then eject it (there's no ejection button, but just choose eject from a right-click on the sidebar).
I scored about 9800 at stock on Geekbench with 8GB of 1600MHz DDR3 at stock, (vs. 9600 with my previous H67MA-UD2H and the same DDR3 downclocked at 1333MHz), now 11850 @ 4,19GHz.
If you're reading this being a newbie, don't try to be smarter than the guide, click what is required and nothing else. If you like to experience, then expect to try it a few time like I did.
Be aware that it may not work properly with another Bios version, and it won't if you don't set first 64bit-mode and AHCI.
I made two screen captures of my Multibeast settings, but ImageShack isn't user-friendly anymore (the UI seems to have changed for the worst), so if someone want them, please indicate another easy-to-use free service or PM me your email address, as I'm not a regular. Once everything's working, I usually go back to my real life of a French cheese-eater. For instance, since it's working fine, I won't bother with UEFI in order to gain a few seconds at startup.