I've played with the FM 8 trial version for an hour or two. In addition to Sarah's findings, I noticed that (disappointingly) some old interface problems remain. Dialogs like Conditional Text, Variable and Cross-Reference are still not resizable. Variable and Cross-Reference are still modal.
The new 'tab bar' feature looks interesting, but on my 20", 1600x1200 monitor the bar fills up when I've got 10 or so documents open. Most of my books consist of more than 10 documents (and I usually keep all documents in the book open), which makes this new interface as useless as the Window menu for selecting a document. This means I'm still reduced to making sure the book file is always visible, so I can select a document by double-clicking it in the book file. That's three clicks (plus making sure the Paragraph and Table Designer are closed, to avoid that bug) for something that could have been a single drag action from the Window menu. Harro de Jong