At 500 points, you can request a $500 certificate to be applied to your cruise. I've redeemed $2,000 worth of points so far -- and it's definitely helping fund my cruise addiction!
The upgrades are available starting at 250 points, but the general consensus is that it's a waste of money to use them for upgrades. Let them build up until you've got at least 500 to redeem for the certificate.