On further inspection, there are some bummer parts:

1) It takes 4-6 weeks to receive the certificates, so you have to plan early.

2) Short expiration dates: "Radisson and Marriott certificates expire on May 31st, 2006. Avis certificates expire on December 15, 2005. Hertz certificates expire on February 28, 2006."

Also the rules are fuzzy about how exactly to use the hotel rewards. E.g., the Marriott rules make it seem like it covers any guest room, so you make the "qualifying" (an undefined term) reservation for any guest room (no mention of certificate necessary), and then upon check-in show the certificate. What happens if only an upgraded room or a suite is available?