I have one that I've been playing with for a couple weeks. My use for it is an all-in-one Proxmox server running a NAS, router, Plex, and probably some other misc services like Unifi and Pihole. I'm still experimenting, but it's been rock solid so far. I've got passthrough working for an HBA and a Quadro P400. HBA gets passed to either FreeNAS or OMV. The Quadro will go to either a Windows 10 or Linux VM.
I don't know about SLI as I don't have two capable Nvidia cards, but the top and bottom pcie slots can run at 8x which is how I have it configured right now. The top slot also supports bifurcation into quad 4x. I'm toying around with using that to get a total of 5 pcie 4x slots, but that's probably on the backburner.
Here's the setup as it stands currently:
2U short-depth case (~15" deep)
Ryzen 1700
Wraith Stealth or Dynatron A24 cooler (still testing)
64GB Samsung ECC ram (2133 atm, will try to overclock soonish)
LSI 9200-8e HBA
Mellanox Connect-X3 Dual 10g NIC
PNY Quadro P400
2x SM961 256GB (passed through for ZFS cache)
WD Blue 3D 500GB for Proxmox host
*edit* Also, they're in stock at newegg as I type this. They seem to disappear pretty quickly though *edit*