Hello. I have Sonar 8.5.3 running fine in Win 7 Ultimate 64 bit. I'm getting another machine. I planned on getting Win 7 Ultimate 64 bit to be sure it works, but have considered Win 8.1 Pro 64 bit. Is anyone or anyone you know using Sonar 8.5.3 64 bit in either Win 8.0 or 8.1 64 bit? I might as well make the move to that if it will work. I've heard most Win 7 64 bit programs and apps work fine in 8.0 or 8.1 since it's based on 7 (and not as big of a change as going from, say, XP to 7 or 8). But I don't want to learn the hard way I need Win 7 instead. I'd rather get the right OS for that to begin with. Thanks.

The real issues people have had is making sure all the audio hardware has good driver support for Win 8. If yours does, you should be fine. At the same time, I'm not using 8.5.3 with Win 8 and hopefully someone else using it will chime in.

If running Sonar v8.5.3 and you plan to stay with that version, I'd recommend not moving to Win8.1.
It may run just fine... but v8.5.3 development is done.  If a problem crops up, there won't be a fix.
Best to go with the "sure thing" with Win7x64...
 
As Mesh mentioned, Win 8.1 is generally working fine... as long as your audio interface/driver is solid.
In our tests, we find performance to be equal to Win7.
If you want the ability to use a multi-touch monitor, you're essentially forced to go with Win8.
 
 

The only device I have that could not be installed in Win 8/8.1 native mode was my Frontier Tranzport.  It had to be installed in a compatibility mode.  The Tranzport works fine in 8.1.
 
My main system's motherboard had to be sent back to Gigabyte for warranty repair.  I set up my backup system, that has 8.1 on it and I didn't miss a beat while the Gigabyte motherboard was gone.  FWIW it looks like Gigabyte just replaced the motherboard as the board I received does not look like it was repaired.

Thanks for the responses. I have an M-Audio ProFire (FireWire) interface. That should be ok. I'm sure I'll go to a Sonar X version eventually. For now I like the appearance and the more compact toolbar of Sonar 8.5.3. The extra features and engine in the X series is great. Once I've gotten past this upgrade, I can look further into that, to see if I can handle it.

I would not upgrade to Windows8 with a FW interface until you are absolutely sure it works! 
 
AFAIK, the FW support have not been very well incorporated in Win8. 
 
There are a few "workarounds" which you can find at the RME forums (I'll post it later today) 

That's huge Kristoffer, thanks. I'm really tempted to stay in Win 7. I figure that's got at least 5 years of future update support from Microsoft ahead of it. I think the drop of support for XP will have people flock to it's more XP like style than 8.1. XP got a dozen years support, I have to think Win 7 will get at least half of that. I'm used to it. I'll never do touch screen stuff in my studio or on my portable rig. Thank you all again. This place had been one helpful forum. 

A bit late, but here's the link
http://www.rme-audio.de/forum/viewtopic.php?id=15505
 
For the record, I'm using my FW1884 (Tascam interface/controller) with Win 8.1 (and X3 Trial - for now) and got it working nicely. 
But: I have to reboot my DAW every time without driver signing enabled. A bit of pain but I don't have to restart it very often now :)

A person can look up the lifetime of an OS. XP is just winding down on April 2014. Windows 7 goes to 2020.
11 years is about right.
 

update: I've managed to remove the annoying "driver signing" reboot
 
:)