Hello Clinton Gallagher,
Thank you for your interest in JustDecompile.
To answer your questions, Yes, you can use JustDecompile to open Windows 8 assemblies.
I have made a sample video, demonstrating how to open Windows.Web.Syndication. You can see it here.
This particular *.winmd file doesn't contain code, so that's why you see only signatures.
Should you have any other questions, please don't hesitate to contact us.
the Telerik team