While Sigil has a fully functioning metadata editor, I liked the table-based design of earlier versions better.
Here's a mockup of a table-based metadata editor (based on my
epub2 metadata plugin) that I'd like to see in a future version:
(The Title File-As box should be hidden for epub2 books.)
I know that it'll be difficult to create a solution that works for epub3 and epub2 books, but especially new Sigil users will probably prefer a table-based GUI over a tree-based GUI.