If you don't spend a lot of money on the card, the value of your rewards might be less than the annual fee for the card, and you would lose money overall.

In terms of flexibility, some of the American Express charge cards allow you to transfer points to a selection of different airline mile programs. You won't get a big point bonus for signing up, though. The Starwood Amex can also transfer points to a bunch of different airline and hotel programs.

If you really want to rack up the miles, see if you can pay your rent/mortgage with the card. If you pay the card off right away, you will be getting free miles for money that you have to spend anyway.