Geo524 - Are you able to use the 1.9b10/3.2.2 drivers and work in Sonar?  I couldn't get it to work at all.  I noticed you're using an AMD processor.  I'm using an Intel i7.  I wonder if the chipset I have is an issue.
 
Riojazz - I noticed you're using an i7.  What motherboard are you using?

aspenleaf
Geo524 - Are you able to use the 1.9b10/3.2.2 drivers and work in Sonar?  I couldn't get it to work at all.  I noticed you're using an AMD processor.  I'm using an Intel i7.  I wonder if the chipset I have is an issue.
 

 
Hi Aspenleaf. Yes. Everything is working. Just a little more CPU intensive than when I had the 1.8 drivers. Glad you were able to get the beta drivers working. I would seriously contact Focusrite and hound them about releasing an official win 10 driver for the 18i20. Power in numbers. The tech I am in touch with is sympathetic to the frustration this is causing and said he is going to foward my emails to the developmental team. It's a shame cause the 18i20 is really a fine interface.
 

Good idea to install Win 10 as a dual boot. I'm not having any major issues with the beta drivers so I can wait as well. 
 
Now I'm wondering if I should install our new printer? All the computers in our household are running Windows 10.

I had some issues when I first upgraded to Windows 10.  I sent an email to Focusrite and a couple of days later they sent me a link to a different (beta at the time) version of MixControl.  It fixed my issues and has worked fine since.

Have any of you run the Latency Monitor that Focusrite techs ask you to run?  I'm getting very strange results.  It says my computer isn't capable of playing audio, and it shows my i7 3.4 Ghz cpu as running at 2.2 Ghz, even though I have disabled the cpu speed controls in the BIOS and in Power Options I have set the cpu min and max to 100%.  CPU-Z shows the cpu as 3.4 Ghz.  I've disabled my wi-fi card and the onboard audio, turned off Windows sounds and all of the other tweaks that are recommended.  The tech told me to update the USB drivers, but the only available drivers for the motherboard are the chipset drivers, and I've already done that.  The USB 2 drivers shown in device manager are Microsoft drivers from 2006.  When I have device manager check for updated drivers, it says the best drivers are already installed.  Any advice?

Yes, I ran it with Sonar running the most complex project I'm working on, per the tech's request.  This file runs fine on Windows 7 with the 1.8/2.5.1 set at 3.0ms buffer.  I can get it to run without pops and clicks on Windows 10 with the 1.9b10/3.2.1 or 3.2.2 set, but I have to set the buffer to 1024 which results in a 10.8ms buffer.  I should probably reinstall the 1.8/2.51 set in Windows 10 and check Latency Monitor with that.
 
When running Latency Monitor without Sonar running, it says there are no problems.
 
I'm using most recent BIOS update that is available for my motherboard.

I just received instructions to delete the Scarlett 18i20 from Device Manager, uninstall Scarlett MixControl, and install a file they sent me, 1.9b10.  Everything seems to be working correctly.
 
Help, About now shows this:
Scarlett MixControl 1.9.64.10
Driver 3.2.285 API 3.0
Firmware version 634.
 
I was never instructed to run Latency Monitor, but I'll give it a try now.
 

I reinstalled the 1.8/2.5.1 set and was able to play the same Sonar project, but this time was able to get the buffer down to 3ms.  This is the same size buffer I used on Windows 7 with the same driver set.  Latency Monitor now said my system was suitable for audio...  Unfortunately, these are the drivers that became unstable for me on Windows 10 and prompted the need to try the beta drivers.  I'm going to do some testing to see if the other changes I made to my system made it more reliable.

riojazz
Latency Monitor seems to be higher than I remember it.  Once I started SONAR with a project, it shot into the red.  Again, I don't remember things being bad before.
 

Same results here riojazz.

Good news. The latency settings and how you select them is what I liked best about the 1.8 Mix Control. Have you noticed, are you using less CPU as well?