The expected release is summer of this year. You can get preview copies of Win10 now. To qualify you are supposed to be a member of Windows Insiders.
You can find out more about the current release here: Windows 10-10122.
Those running it and Second Life™ are having problems. It is hard to tell what is going on as I can only find bits and pieces of people’s conversations as they try to resolve problems.
I am holding off going for Windows 10 until more people are using it with Second Life.
Windows 10 will be a free update and the last NUMBERED version of Windows. I have mixed feelings about that. My phone recently updated and I hated the result. All sorts of things that worked well where changed. Buttons I r4ecognized changed appearance and location. Very annoying. I am a year from being able to root my phone and take control of it.
I suspect we will start to see many of the same things happen with Microsoft’s incremental updates to Win10. While I look forward to Win10, it is with some trepidation too.