My thought is that I only have one proper credit card, and this is sometimes bad. So I am looking for one as a back-up card. I'm mainly interested in having something that does not have an annual fee, and will not have a hissy-fit if I don't use it very much. I don't really care about the interest rate, since I won't be carrying a balance on it. Cards with nifty extras (like points cards or something) are prefered, as are ones that are more widely accepted internationally (i.e., Visa, Mastercard -- no Discover card!)
On a related note, lucifie, here is a link to the milage card that I have. Two important things to keep in mind with this: First, it's with United. Other airlines do have their own card, if you would prefer not to deal with United. The other thing to keep in mind is that it has a $50 annual fee. Depending on how much travelling you do, this is worth it. So, my England tickets were paid for with this. But if you don't do that much flying, then that fee may start adding up to being more than your annual travel costs would be anyway.
Oh, if you do get a milage card, pay for EVERYTHING with it. Gas, groceries, bills, anything you can throw on the card, do so.