It looks like a placeholder (see welcome screen below.)
It's not really Windows 10, it's a version of Windows 10 released as part of the Windows Developer Program for IoT - there is no desktop, there are no tiles, no apps.
So, what can you do with it?
You can develop applications on a Windows 10 box using Visual Studio 2015 Preview and transport them to the Raspberry Pi to run.
I've loaded Windows 10 in Parallels on a Mac Mini at home to experiment with it - it was relatively easy to set up. I won't be doing this after VS 2015 goes live because of the price tag, so this is just a temporary environment.*
There's a remote debugging facility, you can write your apps in C# or C++ and you have full access to all the general purpose IO pins on the Raspberry Pi.
When all the pieces get to production, it looks like this will be a fine, albeit somewhat expensive, development and debugging platform for folks who don't like Linux and open source tools.
For much more information, instructions and demos: http://dev.windows.com/en-us/iot
Much more to come!
*I've heard that the Visual Studio 2015 Community version also works but I haven't tried it yet. This version is much more in line with my budget (it's free...)