All of the above...

Build 10240 is in the slow and fast rings now. I've installed it and it's a very polished release. If this isn't the RTM release, it's very, very close.

According to posts on the Windows 10 forum, 10240 is the "RTM" version.

Yes, I've seen those posts and they are probably right. I also wouldn't be surprised to see one or two more builds before RTM, but given the lead time OEM's require for a new Windows release, this is likely the one.

Until M$ releases it or says it.. It's just speculation right or wrong.

I've been using Win10 on my business machine for a few months now and varying versions have worked at varying degrees of "goodness".  For the most part, it's been fairly straightforward and clean.
 
I tried an earlier version in (maybe) January of this year and when I tried to get my PCR-800 working it was a complete no-go.  I'd be interested in the exact edits to the exact file that are needed.  I'd like to get Win10 on this computer for music but not being able to use my PCR-800 make my music making nearly impossible.  I do pretty much everything with it.  Without the installation even getting to the point where it will recognize that I've even plugged in the PCR, I can't even get it to power on and pass midi through - so it doesn't work AT ALL.
 
Any help appreciated on this.  I'd give an upgrade another go if I thought this would work - everything else on my machine was "good to go".

I have a PCR-500, which probably is the same as far as computer connection.  I have the 10240 Win 10 release and I'll give it a try later today and report back on how it does.  Everything else I've done with Win 10, in relation to recording works exactly as it does in Win 8.1. 
 
I've only used the PCR-500 on Win 7 and not on my 8.1 installation.  I don't do much MIDI and thus rarely use it.  Just bought it when Guitar Center was "blowing out" the discontinued line as I didn't have a MIDI keyboard.

Tried my PCR500 in Win 10 and it does not install.  I have the Roland Windows 8.1 driver (and it installed OK in Win 8.1 64 bit).  I tried Windows 8 compatibility mode and no go (there was no option for Win 8.1).  When I tried it in a Win 7 compatibility mode it would not install and the Roland installer noted incorrect Operating System.
 
When I upgrade to Win 10, it will only be my Win 8.1 install on a dual boot system with Windows 7.  If I want to use the PCR500, I can use it with Sonar/Win 7.
 
 
 
 

Did you try to figure out what CadErik suggests in "hacking the driver file"?
 
CadErik: I took a look at the downloaded driver INF files and I didn't see anything in there that was "hackable" in terms of the OS version.  Can you clarify what you changed in which files to make this work? 

Talking to some people on the Windows 10 forum, they suspect its some oversight in Win 10 as Win 8.1 compatible drivers/hardware are supposed to work.