Hi,
One thing I can say is that their interface is not as good as you would think. They also have a more technical interface much like OpenACS. Although maybe due to their design they have a unified editor that is able to edit almost all of their contents. I think this editor also gives users the power to relate one object to another. The one that I have seen that has a good interface is opencms. It really puts my work in modetp to shame. But I think opencms is too specific for its tasks.
You can check out the screen shots of mmbase editor here:
http://www.mmbase.org/mmbasenew/index2.shtml?about+1069
I have tried the editor myself. It does not have a fancy interface but it sure can edit almost all objects in mmbase plus adding relationships. On the post URL above you can also see the apparent design of the system on the left hand right hand side. Those images, people and urls appeared there because it was related to that article.
So as I see it mmbase approaches the problem of cms with different objects that can relate to each other. When an object is pulled it shows also other related objects.
OpenACS ETP approaches it to be at this context want content will I display. Then according to the template it may show objects at the same level, lower level or higher level on its context.
Both approaches has its merits and uses.
