Typically, the airline credit cards will earn 1 frequent flyer (FF) mile for each dollar spent on the card. (cash advances/balance transfers, etc usually don't accumulate FF miles. The cards work best if one is on a cash basis--pay off their balances each month. The interest rate is usually higher than other cards. There is usually an annual fee on the airline cards.

The UA one is not bad because there is a 15K/20K signup bonus. (the 20K bonus is a link on one of the flyertalk threads). Even if someone spends only 5-10K in a year, one could get a free domestic ticket (20K signup plus 5K spending) for the $60. The United airlines card in that case (say 5K spending/year) is not worth carrying for more than a year. The airline cards have an annual fee of $50-100 depending on which card, etc. If one pays the balance off each month & charges alot of things on the card, it makes sense to have a card. If a person is a very heavy user of the card (say around 7-10K+ or more), the card makes sense. Of course, any of the no annual fee cards that earn points may make more sense depending on situation. (Starwood AMEX comes to mind).