I have read that there are a lot of problems to get this to work.
Has this been addressed yet?

Did the BIOS auto recognize or did you have to do anything other than plug it in and load the OS?
 

As mentioned in post 4 it should work without problems.

Hope this helps. 
 
Windows 8.1 and Win 10 should load the correct drivers automatically
 
NOTE:  Remove all other drives except optical, check your boot order

When booting from NVMe on my X99 Classified with an Intel 750 PCIe SSD - it took the following steps.
 
NOTE:  You MUST do ALL 4 Steps - per Intel documents - (step #4 is a head scratcher as to why this needs to be)
 
1) Boot Tab ----- Fast boot --- OFF
 
2) Boot Tab ----- Boot mode --- UEFI
 
3) Boot Tab --- CSM Configuration ---- Launch CSM --- Enabled
     a) boot option file ----UEFI
     b) Launch storage oprom ---- UEFI
     c) Other PCI device ROM priority ---- UEFI
 
4)  Advance Tab ----- USB Configuration --- Legacy USB Support ---- Disabled
                                                           --- XHCI ---- Smart Auto (how mine is set)
 
NOTE: other items I see in BIOS

 Boot Tab --- UEFI Hard Disk Drive BBS Priorities (bottom of page) -- Boot Option #1 --- Windows Boot Manager
 
Save & Exit Tab
      Boot Override
      UEFI: Built-in EFI Shell
      Windows Boot Manager (Intel SSPDEDMW400G4
 
http://www.intel.com/cont...-drives/000005967.html
 
After setting BIOS --- then I loaded Win 8.1 Pro 64 bit, OS loaded from USB thumb drive. 
Upgraded to Win 10 Pro 64bit, activated, then clean install of 10 all from a USB drive
 

I appreciate the former helpful replies. However, finally building my dream system and I know it's overkill but, it's fun. New 170 MoBo and 2 new Intel 750 series AIC. Titan X in slot 1, AICs in slot 2 and 3. Trying to configure them in RAID 0. I've spent the last week doing extensive research and 2 days trying every conceivable BIOS config and have found no solutions. Appears that outside of the purchasing the M2 adapter and exchanging to the 2.5 inch "case" SSD's, there is no guidance on setting up RAID for the dual AIC SSDs. Intel, ASUS, Tom's Hdwr, DIYasus, and many others just do not address this RAID 0 issue with the dual AICs. I also get the impression that without the adapter, even though the 2 slots are Gen 3, the performance could be less than designed. The BIOS is updated and I only have an external DVD drive connected. Although the 2 drives appear in the BIOS, when setting up RAID I receive a notice that there are no drives detected. I have just enough knowledge to make me dangerous so, any help would be deeply appreciated. (Is it possible I have to format or install an OS on one of the drives and then go back to the BIOS?) Thanks in advance for your thoughts and suggestions.

RAID between 2 separate PCI-Ex devices is not possible. 
 
Intel 750 will work in Windows pseudo-RAID but not as bootable drive. You can configure Dynamic Disk in Disk Management to any RAID level you want. Technically there is no point of using RAID0 (except recording like 20 4K streams at once) which is not RAID anyway as there is no redundancy involved - that's R in RAID. CPU will never handle that kind of IOPS in home environment anyway. The only logical choice is R1 mirror. Not much performance is lost with redundancy bonus, but Windows Dynamic Disk is not true RAID, because it doesn't offer anything that even most basic controller offers. If for example mirror enters degraded state Winblows won't even notify you unless you find that info yourself. Pretty retarded but there you go. My opinion - don't waste time on NVMe RAID unless you running some serious workload like VM server or working with TBs of live video content.
 
Classic RAID will not work because you can't connect NVMe drives (all of them) neither to SATA port or HD SAS SFF-8643 on RAID controllers. While 2.5" version plug 8639 looks similar it's totally different and it may damage the drive if you by mistake/force connect it to 8643 on RAID controller. 
 
Stay away from 2.5" version. Firmware for 8639 controller is motherboard specific and just like with all Thunderbolt controllers it will work only with certified motherboards.

Thank you Ypsylon for your help, sage advice and I agree with the Windows proposition. I've always felt if it can't be done in BIOS, then it's a MoBo restriction.