I have a 12 core HP Z600 running Mountain Lion perfectly (even better than a mac, the only problem was for install you need JMicron36xATA.kext in /Extra/Extensions to enable the internal HDD controler).
A few days ago I found a deal and bought an HP Z820 hoping for the same easy flawless results , and have spent the last couple of days trying to get OSX to work on it.
There are some problems, however some things do work and it's just a matter of trying things and having lots of patience and an SSD drive with many test partitions for quick cloning when it works and rebooting when it doesn't.
I tested with Mountain Lion, Mavericks and Yosemite, and here are my notes.
For speed, I used Unibeast but instead of a USB, I actually used empty SSD partitions for installers and just created a Chimera USB bootloader stick with the latest build.
The Z820 specs are 2xE5-2670 - 64GB Ram - Wifi-TEW624 USB - (I removed all other cards for testing) - My video card is an old HP Quadro FX1700 that works perfectly in my Z600 but it failed to load with full acceleration on the Z820 (my backup video card also didn't work Quadro FX580) black screen or endless spinning wheel, frozen after Bluetooth message in verbose boot. So In order to get to the installers, I used the flags "nv_disable=1 npci=0x2000".
The SAS and SCU controllers don't work, so you have to use the 2 internal grey SATA ports for your HDD.
Bios settings are simple, plain AHCI mode in Storage-Storage Options-SATA Mode (not Raid+AHCI!) and make sure virtualization is disabled in Security-System Security VTx.
Mountain Lion 10.8.5
After Install run Multibeast - Easybeast settings + for Network use Intel kext (I believe only one port works, even though both show up), for audio google and download VoodooHDA-2.8.8.pkg, Firewire shows up in system profiler, but can't test it. USB3 works using GenericUSBXHCI.kext. System Definition use MacPro 5,1. ***** Not working SAS+SCU (Processor shows as Unknown)
Mavericks 10.9.5
After Install run Multibeast - DSDT Free settings + Drivers Trim Enabler, All FakeSMC + HW Monitor works! + VodooTSCSync 8 core for Network use Intel kext (I believe only one port works, even though both show up), for audio google and download VoodooHDA-2.8.8.pkg, don't use KernelCache, for System Definition use MacPro 6,1. Insert the following in smbios <key>SMoemcputype</key> <string>2561</string> so it recognizes CPU Type. ***** Not working: USB3, SAS+SCU, Firewire
Yosemite 10.10
To install, had to use boot flags ' nv_disable=1 npci=0x2000 cpus=31 -x -f ' or else I get kernel panics galore.
Tried similar settings to Mavericks on Multibeast, but Yosemite runs much slower than other 2, and since most kexts don't work, + no wifi drivers for my USB dongle, no USB3, SAS+SCU, Firewire, etc... I Gave up on this version. (It also froze every 3-4 boots at login screen)
Looks like I'll get a better Video card and keep working away at Mountain Lion and Mavericks.
I hope more people buy these fantastic machines and support gets better, since they are faster than the best MacPros, and a lot cheaper (if you buy on ebay), I guess the problem is that they used to be very expensive machines, and their motherboard is rare, so a few things need to be solved still.
I managed to extract the DSDT using windows (all OSX apps crash on extract), but I don't know enough to patch it on my own.
