Unfortunately, in it's present state, FontBook is a simplified version Suitcase
You can create your own sets (i.e. Projects)... but cannot create a subset (i.e. ProjectNo345_WebSite). I suppose you could add the project sets at the top level.
Also, I didn't seem to get auto-activation when I added a new font (in apps such as Photoshop).
But, it is clean and fast... maybe it'll be improved upon by the time of the release.