That good conversion rate is not available for United or Continental miles. Last I looked, the rate for those airlines was 2 points gets you 1 mile. The card is great for most other airlines, however. But it has two drawbacks:

• It is an American Express card. Many merchants do not accept that card.
• The bonus for first purchase is not as good as many of the airline cards.

Do take a look at the Credit Card section of my website below. There you will find some of the best offers available for airline cards (and the Starwood card). American, and United Airlines, and U.S. Airways, for example, have some super up front miles offers for first purchase or certain minimum dollar amount purchases (15,000 - 25,000 free miles are typical). Many are free of annual fee for the first year. Then just cancel it before you have to pay the second year fee. Look in both the Free Card and Annual Fee card sections. Many of the best offers are in the latter section, as they are free for the first year, even though they have an annual fee.

There is not reason why you can't carry more than one card. I carry four, one for gasoline, one for 2% rebates on certain purchases (as I won't pay more than 1 cent/mile for miles), the Starwood card, and whatever airline card Visa or Mastercard gives me the miles I most need now.