You may have noticed the RC channel Blue Steel is running the server side of the Materials System update. While there is no Project Viewer available yet, this suggests that it is getting close. What more confidently suggests the nearing release are these screen shots of Materials in use.
The above image shows an interesting surface. I think that is a simple cube with a normal map.
This is an obvious simple cube. The Specular properties have changed to make it appear a matt texture.
You have likely seen this object before. It is the one used in the original announcement of the project.
These images are from Geenz Spad, the lead on the Materials Project.
According to Geenz, most of the render pipeline is ready. Also, the User Interface is complete except for some polishing. So, we can expect to see a Project Viewer real soon. I think that means 2 to 4 weeks. But, there could be surprises that slow things down.