Here is some information that can answer your questions:
Sustainability
OpenFaces has a 4-year history of product use since the first version of QuipuKit 1.0 was released, so it had time to polish.
Compatibility with other libraries (PrimeFaces, RichFaces, etc.)
In addition to OpenFaces, today there is only one JSF 2.0 library in production (November 5, 2010) - PrimeFaces. The latest PF 2.2.RC1 has a conflict with OF 3.0 Ajax, although this should be resolved in the near future - see this request for the status of this problem.
As for RichFaces, its version compatible with JSF 2.0 is in a preliminary state, and the latest version of RichFaces 4.0.0.M3 can be successfully combined with OF 3.0 with very fast testing.
Ease of use
Installation is as simple as adding cans to the application, and using is not more complicated than using standard JSF components. The API is essentially similar to standard components, but there are many more configuration options. All advanced configuration options are usually optional, and you can keep component declarations simple.
skin capabiltites (theme support, etc.)
There is no special skin system, but all components usually have many settings to customize all their aspects of appearance.