Interface is messy, you will find lots of undocumented, not implemented stuff inside, look trough the source and try it out see what works the best
True, although Javadoc is in the way of being completed.
Two years ago performance was pretty good in comparison with Java3D for example, but some recent benchmarks show that Java3D is way faster these days.
IMHO, a single benchmark with only static geometry and (now) outdated versions of the libs cannot be completely trust. Some patches have been made for improving performances, and using the Alternate vecmath lib seems to provide much better performances (@see Yuri).
Very little community effort in last year and a half, you will most likely end up fixing and adding stuff on your own.
This is ignoring work from William Denniss, Arne Müller, David Wallace Croft, Lilian Chamontin, Yuri VI. Gushchin, myself, and a bunch of others I forget here..
No generic GUI mechanisms
Please be more explicit about what you mean by "generic GUI mechanisms". A Xith GUI project has been started recently (1-2 months ago) by someone.
Missing some more advanced culling mechanisms (BSP, Portals, OctTree, Occlusion....)
Occlusion is implemented and working, classes for OctTree exists, but I don't know if it works.
For example I think that once upon a time Xith had shadows of some sort implemented, I have seen some interfaces and classes in there that might be used for that but we have never been able to use this feature.
RenderOptions options = new RenderOptions();
lines 112-116 from org.xith3d.test.MD2Test.java
Not explicit I admit, but it works. I'll howto that.
(Note : cp is CanvasPeer