Here's some info that might answer your questions:
Stability
OpenFaces has a 4-year production usage history since its first version named QuipuKit 1.0 was released, so it had some time to get polished.
Compatibility with other libraries (PrimeFaces, RichFaces, etc.)
Besides OpenFaces, there's only one JSF 2.0 library in production state as of today (Nov, 5 2010) -- PrimeFaces. The latest PF 2.2.RC1 has a conflict with OF 3.0 Ajax though, which should be resolved in the nearest future -- see this request for the status of this issue.
As to RichFaces, its version compatible with JSF 2.0 is in a prerelease state, and the latest RichFaces 4.0.0.M3 could successfully be combined with OF 3.0 upon a very quick test.
Ease of use
Installation is as simple as adding jars to the application, and usage is no more complex than using the standard JSF components. The API is in essence similar to the standard components but there are a lot more configuration options. All advanced configuration options are generally optional, and you can keep component declarations simple.
Skinning capabiltites (Theme support, etc.)
There's no special skinning system, but all components generally have many settings for customizing all of their appearance aspects.