Cool - I guess the usefulness for me is limited as:
(a) All my projects are version controlled web apps
(b) they all live in ~/Work/$appname/trunk
(c) They're all rails or rails-y in directory structure
(d) I tend use subversion to manage the state of my projects, so don't need tabs to stay open
(e) I worry about leaving ._DS_Store files everywhere, so tend to stay out of the Finder
None of this means "project files are crap", of course, but any time I see a feature I'm not using, I get excited that maybe it could be useful :)
Anyone using project files in a similar environment?