2 downsides I found:
1) It takes several weeks to get the chrome bucks when you call to redeem them, so plan real early
2) They have a limited amount of time to use them once you get them (like 90 days). Then they are void.
They dicked me around the last time I went to get them, so I moved to another type of card and closed that one.
Just as an aside, with ANY rewards card, its the vendor PAYING your rewards. So when you use these cards, whoever you're buying stuff from is paying higher percentages to accept that card from you. Being a small business man, these costs add up quick. I'd like to see all the rewards cards get canned, and would easily consider getting rid of mine if I did more small guy business.