My experience with ESXI 5.5 and the motherboard P9X79 PRO (socket 2011, 64gb ram), tested with a windows8 virtual machine, is:

-In Esxi 5.1 the machine would freeze for several minutes when booting, that didn't happen in 5.0 and it's been fixed in 5.5.

-Pass-through devices don't disappear when rebooting (I rebooted several times, tested with all the devices)

-The unofficial driver for the nic 82579V (net-e1001e-1.0.0.x86_64.vib) works fine. Vsphere client is crashing a lot (it didn't happen in 5.1), and I think it's related to the network, or maybe it's because I'm only using 1 nic. Because of that I couldn't test this in passthrough.

The following is the onboard hardware of this motherboard:

• Onboard usb 3.0 (ASMedia ASM1042 SuperSpeed USB Host Controller): passthrough works fine with both of them, the virtual machine is able to see them, no errors and it can use the devices you connect (tested with a pendrive and a keyboard)
• Onboard usb 2.0 (Intel C600/X79 series chipset usb2 Enhanced Host Controller): the device shows up in the virtual machine but with a yellow exclamation mark and can't be used. When not in passthrough, connecting devices to the host and adding them to virtual machines works fine (this can't be done with HID devices, as usual). In this mode you are able to add the onboard bluetooth to a virtual machine, and it works.
• Intel Pastburg 6 Port SATA ACHI: I can't test this in passthrough because I installed ESXI on it. It worked in 5.0 and 5.1, so it should be fine.
• ASMedia ASM 1062 Serial ATA Controller (esata): the virtual machine is able to see it, but with a yellow exclamation mark and can't be used. It doesn't work. When not in passthrough, the host doesn't see it and can't be used.
• Marvell P8P67 (sata): the virtual machine is not able to see anything. I tried with a debian virtual machine, and it gives a kernel panic when booting (the vm). Not sure what's wrong with this one, I've read it may be a bug in the hardware/firmware.
• Intel C600/X79 series chipset High Definition Audio Controller: passthrough works. I was experiencing freezes and the sound was barely playing even after installing the drivers. After removing all the other devices, it started working, but the sound was a bit choppy. Then I disabled speedstep and all the power management, attached a keyboard and mouse using the usb3 passthrough and added a videocard. The sound was perfect after that. Not sure exactly what set of changes fixed the sound, but the important thing is it can be fixed.

Also, my Raden HD 5870 works fine in passthrough. Tested with a game and the performance was great, with no errors at all. Video acceleration doesn't work in firefox/ie (but works in chrome) when playing a flash/html5 video with acceleration on. This may be related to the virtual svga.

To sum up, the marvell and esata controllers don't work. The nic, as anything with unofficial drivers, can stop working in future versions. USB3 works. USB2 doesn't work, but it can be used with the host, with the knowledge that no keyboard/mouse can be added to a virtual machine.

Edit: if the vm uses EFI (version 8, I don't know if this is important), esata and usb 2.0 work, although the usb is giving me problems. Read the following posts.