I use it alot on my powerbook when travelling. It's an extremely handy feature, great for anytime you would have a problem with disk space in full res (carrying firewire drives around in the sahara isn't that practical)
Use it to rough edit in, leaving in shots that your not sure about, and then uprez. Ken Stones site has loads of useful stuff on it:
http://www.kenstone.net/fcp_homepage/fcp_homepage_index.html
Powerbook 17 1.67 512meg 100gig FCP HD