With the graphical menus you will notice a slight delay once the "submit" button is clicked. This is to allow for a quick display of a status message, however I'm finding this delay to be annoying and may eliminate it altogether to speed the use of the new GUI.
Also note that with gambrel attic trusses the non-graphical geometry menu is still an option if you turn off the GUI flag in the global settings. For some people this may be easier but I think the new preview pane is a major design asset when it comes to this type of roof.
I'm thinking it might be useful to implement this same type of preview/GUI for regular attic trusses so that one can preview the configuration before committing. Now that I've been down this road once it won't be such a struggle. Most of the other truss types are fairly simple and a preview image is probably not warranted unless the users say otherwise.
I feel that the preview window goes a long way in helping explain what each term is in the input geometry menu but I may also add in some pop up windows (?) that display diagrams or other explanatory notes to help the less educated users.