If you are not averse to risk, want to be part of the Windows development process and get free Windows for pretty much ever after, there is a program for you.
 
Microsoft updated its policy on free Windows 10 installs for Windows Insider members. The Windows Insider program makes prelease builds of Windows available to members on a regular basis and allows you to provide feedback to the development teams and program managers on new features and fixes as they roll out of the dev shops. The payback for this considerable risk is that you get Windows 10 free for as long as you are a member of the program.
 
Microsoft also announced that the program will continue after RTM for follow-on releases into the foreseeable future so this looks to be a great way to help drive the direction of the product and get freebie Windows for a good long time. Check out the program details at https://insider.windows.com/
 
Here are the basics:
 
If you are running a Windows 10 instance as part of the insider's program, you will receive the RTM version as part of a regular "flight" update through Windows Update. There is no charge for the OS for Windows Insiders. Their are only two caveats; first, it is an update rather than a clean install, but you will be able to download an ISO file to do a clean install after the update if you desire. Second, you need to be registered as a Windows Insider with a Microsoft online account on the PC that you will be installing the preview.
 
So in practical terms, this all means that to get the Windows 10 RTM update you have to have a preview version installed on your machine, connected to the Insider program with a valid Microsoft online ID. Assuming you want to do all that, you will:
 
1. Get a Microsoft online account (mine is through live.com but Hotmail, outlook and MSN accounts work too from what I gather.)
2. Sign up to be a Windows 10 Insider (google it, it's right at the top of the list)
3. Make an image backup and a recovery disk (just in case, right?)
4. At the Windows Insider hub, choose to install the preview
5. The preview build will install through Windows Update
 
Please refer to the Insider home page for details on what the program is all about.
 
This program clearly isn't the thing for you if you depend on a stable OS platform for your sanity or livelihood, but for those of us who do this for the grins, it's a great way to participate and get something for it. YMMV, etc.
 
I should also tell you that I am not an employee of Microsoft (though I used to be) and I have no affiliation with the company and receive no compensation for anything from anybody connected with Microsoft for anything whatsoever. Full disclosure, I do own a tiny software development company that targets Windows platforms.
 
Good luck!

Been using it....rock solid for me...quite like it...
All my programs still function normally as well...
Quite impressed....

I've been using it in a virtual machine for development but haven't tried it on my DAW box yet. It's good to hear that you've had success with it. I'm looking to install it this week.

I just moved to Windows 10 on my main machine, build 10130. Aside from Hyper-V breaking the boot sequence, everything else went great, but I did have to switch to VirtualBox for my virtual machines.
 
Sonar also runs fantastically well and my MOTU 8pre works very well, the drivers installed without any issues at all. My system has an NVIDIA GT430 display adapter and the driver installer would run but after rebooting it was still using the default VGA drivers. Fortunately NVIDIA has made Windows 10 drivers available on their support site, and these installed and worked fine.
 
One huge plus - on Windows 8.1 my EVGA X58 Classified III system with i7/980 and 24GB of RAM ran with a DPC latency average of around 470us with spikes over 1000 as reported by LatencyMon. Part of this is due to all the other crap I do with the machine so I just accept the hit, but after installing Windows 10 my machine is averaging 110us with no spikes at all. Now this could be due to something being broken in my 8.1 system configuration, but whatever it is, I'll take it.
 
I'm on the fast ring for updates so I'll probably have some real issues as we close on RTM, so creating system restore points are a bit of religion for me right now, but I'm really liking the responsiveness. I'm currently working on a tune while a virtual machine running Windows 8.1 is installing 128 updates via Windows Update and while Visual Studio 2015 RC is installing on my Windows 10 OS. Not a recommended best practice! A few audio engine dropouts here and there (no wonder!) but it's pretty damned smooth.
 
I'll post more once I get the thing in a stable state.

kevinwal 
Microsoft updated its policy on free Windows 10 installs for Windows Insider members. The Windows Insider program makes prelease builds of Windows available to members on a regular basis and allows you to provide feedback to the development teams and program managers on new features and fixes as they roll out of the dev shops. The payback for this considerable risk is that you get Windows 10 free for as long as you are a member of the program.

How is this different to getting Windows 10 free just by upgrading within the first year?  (other than getting the buggy beta releases?)

I have been getting and testing the pre-release versions for quite a while.  I have the 10130 build installed and have Sonar Platinum and my recording hardware installed (I don't use it for audio production just testing). 
I am also active on the Windows 10 forum.
 

I'm thinking it might be time for a backup.

Karyn

kevinwal 
Microsoft updated its policy on free Windows 10 installs for Windows Insider members. The Windows Insider program makes prelease builds of Windows available to members on a regular basis and allows you to provide feedback to the development teams and program managers on new features and fixes as they roll out of the dev shops. The payback for this considerable risk is that you get Windows 10 free for as long as you are a member of the program.

How is this different to getting Windows 10 free just by upgrading within the first year?  (other than getting the buggy beta releases?)

This will work if you do not have an upgrade eligible version of Windows already. The insider version is available to people who do not have a licensed copy of Win 7 or 8. So if you get the free pre-release version, and then convert it to a fully licensed version of Windows 10, you have gotten it with no $$$ cost to you. The free upgrade from Win 7 or 8 implies that you paid for Win 7 or 8 at some point. 

slartabartfast

Karyn

kevinwal 
Microsoft updated its policy on free Windows 10 installs for Windows Insider members. The Windows Insider program makes prelease builds of Windows available to members on a regular basis and allows you to provide feedback to the development teams and program managers on new features and fixes as they roll out of the dev shops. The payback for this considerable risk is that you get Windows 10 free for as long as you are a member of the program.

How is this different to getting Windows 10 free just by upgrading within the first year?  (other than getting the buggy beta releases?)

This will work if you do not have an upgrade eligible version of Windows already. The insider version is available to people who do not have a licensed copy of Win 7 or 8. So if you get the free pre-release version, and then convert it to a fully licensed version of Windows 10, you have gotten it with no $$$ cost to you. The free upgrade from Win 7 or 8 implies that you paid for Win 7 or 8 at some point. 

Ahh, so for people still on Vista or XP or OSX.
 
or for people building their own system that need an OS??

The Insider program is for anyone that wants to participate, regardless of what OS you're on, but yeah, if you're on OSX, Vista or XP this is a way to get 10 free without having to upgrade to 7, 8 or 8.1 first. You can download the ISO, make a bootable DVD or USB device from it and you're off to the races.
 
I should point out that it can be a frustrating experience especially if your machine is used for a variety of purposes. For example, I run multiple virtual machines supporting my software development business and use the host OS for development and as my DAW machine, so the surface area of the OS I hit on a day to day basis is quite a bit larger than if you did just one thing with the box, so the opportunities for issues and show stoppers is much larger. I have in fact run into several issues with Hyper-V and found some interesting issues with using system restore points and "modern apps".
 
That being said, Sonar Platinum runs great using my MOTU 8pre. No issues with plugins, drivers or latency whatsoever, so if your machine is primarily used for that, you should be fine.
 
Edit: If you go this route, I'd strongly encourage you to use the Windows Feedback tool to post problems and change requests, and to periodically browse other issue reports and upvote the ones you agree with. Microsoft does read this stuff and does respond to them with changes to the OS (although they don't reply directly through the tool, that's not what it's for.)