I've had numerous issues since upgrading from Windows 7 to Windows 10 (x64).  Sometimes Sonar will show that the interface (18i20) is connected, but won't play any sound unless I shut down Sonar, unplug my interface USB cable and re-insert it, then restart Sonar.  Other times it says there is no interface connected.  I have gone through the optimization suggestions, such as turning off power management on the USB hubs, etc, but the problem persists.  I'm currently using the official release of Mix Control, 1.8, but it's pretty old.  Focusrite says it will work with Windows 10, but they do have a beta version they've been working on, but it's not yet an official release.  I'm tempted to try it, but it apparently updates the firmware on the interface and can't be reversed.  So I'm curious if anyone has tried it and what the result has been.

Yes. It's working okay with Sonar on Windows 10 with a couple of exceptions. 1) I noticed a slight increase in my CPU usage since switching over the the beta driver. Not a show stopper but I thought I'd get better performance. 2) I do not like the choices to change buffer settings with the beta driver. I liked the control features much better with 1.8 which also worked well with Win7 and Win10. I only upgraded to the beta drivers because I wanted to stay with the most recent drivers. I can't say I regret it but had I known I would've just stayed with 1.8. Your mileage may vary. One thing I noticed when talking with Focusrite support is they seem unsure as to when the official drivers will be released. I've been using 1.9b12 for almost a year now. I hope they release the official drivers soon.

Were you having any issues with 1.8 and Windows 10?  I didn't have any real problems on Windows 7, but on Windows 10 they are almost unusable.  Also, could you elaborate on the control features of the beta Mix Control and why you preferred 1.8?
Thanks

No issues with 1.8 on Windows 10.

Instead of selecting 2ms,3ms,5ms, etc... you now have Recording/Balanced/Mixing. You can no longer select milliseconds. Only the buffer settings. I liked 1.8 because I had the option of selecting latency settings by milliseconds.
 

Is it not possible to go back to 1.8?  I re-read the description of 1.9b12 and it sounds like you can't use the 1.8 Mix Control with the beta drivers, but it doesn't say you can't go back to 1.8.  It does say you can't roll back the firmware.

You definitely can not use the 1.8 Mix Control software. When I installed the 1.9 driver I also had to install the newer software. I'm not sure if you can go back or not? I would definitely contact Focusrite support to find out before installing the 1.9b12 driver. 

I opened a support ticket at Focusrite, so I'll let you know what they advise.  I did find these instructions for completely removing the Focusrite drivers over at the Avid forum.  They are from a Focusrite support tech:
 
1) With the Scarlett connected please open the Control Panel > System > Device Manager. Locate the Scarlett under Sound, Video and Game Controllers section.
 2) Right click the Scarlett and choose Uninstall. Make sure to check the box to remove software for the device.
 3) You will see the unit disappear from the list. Unplug the unit and plug it back in. It should reappear in the list.
 4) Repeat steps 2 and 3 until the Scarlett appears with a yellow exclamation mark, and/or the properties says Code 10. The Properties > Driver tab will likely say a Microsoft driver is in use. This means all the drivers are removed.
 5) Next go to Control Panel > Uninstall a Program. Uninstall any Focusrite USB Audio Driver, Scarlett Family Audio Drivers or Scarlett MixControl you find here.
 6) All Focusrite drivers are now removed. Please reinstall the MixControl software and/or driver for the unit.

Why does my MixControl say version 3.7?
 
Assuming I haven't accidentally invented a time machine, I'm guessing there are separate versions for USB and Firewire?

aspenleaf
I opened a support ticket at Focusrite, so I'll let you know what they advise.  I did find these instructions for completely removing the Focusrite drivers over at the Avid forum.  They are from a Focusrite support tech:
 
1) With the Scarlett connected please open the Control Panel > System > Device Manager. Locate the Scarlett under Sound, Video and Game Controllers section.
 2) Right click the Scarlett and choose Uninstall. Make sure to check the box to remove software for the device.
 3) You will see the unit disappear from the list. Unplug the unit and plug it back in. It should reappear in the list.
 4) Repeat steps 2 and 3 until the Scarlett appears with a yellow exclamation mark, and/or the properties says Code 10. The Properties > Driver tab will likely say a Microsoft driver is in use. This means all the drivers are removed.
 5) Next go to Control Panel > Uninstall a Program. Uninstall any Focusrite USB Audio Driver, Scarlett Family Audio Drivers or Scarlett MixControl you find here.
 6) All Focusrite drivers are now removed. Please reinstall the MixControl software and/or driver for the unit.

Thanks. One thing I can say is support is excellent. Sometimes it takes a little while for them to reply but they really are excellent.