The user interface in WF5 is pretty much the same as WF4. There are some major interface changes to drawing mode. Otherwise there are just small changes that should be easy to figure out. Oh, There is no resolve mode in WF5, so it just shows the failed components in the model tree and lets you fix them at your convenience. In some was this is better, like it allows you to fix the part/feature that caused the other to fail instead of locking you into fix the one that is the result of the other.
If someone opens and saves in WF5 it cannot be opened in WF4 - unless you go back to the previous version. But, you probably knew that.
Hope that answers some of your questions on user experience. If not just ask again a different way, so I understand better.