Well, haven't got any emails about this yet but be aware that if you have any of the "Must Have Web Driver" cards (780Ti/Titan Black/750/980/980Ti/Titan-X) that you should NOT run this update yet.
Nvidia will doubtlessley have an update in next day or two. So, most folks should just be patient and all will be well.
For those who clicked before thinking and now don't have a working GPU going backwards isn't easy.
With a few terminal commands you can be back up running.
1. Go to S/L/E and drag "NVDAStartupWeb.kext" onto the desktop. Do it again. OSX will warn you that there is already one on desktop. Hit "Keep both" This should result on one with right name and one named with the number "2" after it. The one with "2" is your backup in case things go south.
2. Right click to get inside, use X-Code or a text editor to open the info.plist. Drill down to NVDARequiredOS. Where it shows "15G31" change this to just "15G"
3. Save and close.
4. Use a kext utility to install this into S/L/E.
5. Boot into recovery partition. This can be tricky, even with an EFI Maxwell card. Have a look for the nv_disable=1 trick, it will allow any EFI card to get there.
6. Open the terminal here and type "csrutil disable". You will need an admin password.
7. Change boot args to remove nv_disable.
7. Reboot, all should be well.
Please note, these instructions assume a fairly high level of knowledge. If you don't understand this stuff, just wait a few days for the fix.
UPDATE: Best option is to roll back to "BETA" versions of these drivers. Look on our Driver Page. Find your OS (ie, 10.11.5 or 10.11.6) and look for FIRST driver in list. Easy way to tell Beta from Final is the letter "b" in the name versus "f". (Captain Obvious reported this)
UPDATE #2: Nvidia has kindly released drivers for 10.10.5 and 10.11.6, linked on our driver page. The 10.11.5 driver is corrupt so those folks should keep using Beta driver until it gets fixed.