In general, I don't do store credit cards. I have enough credit cards to keep track of as it is, so I don't see the point of adding to my wallet these store cards that usually seem to have very little to offer (meager signup bonuses and rewards and the ability to use the card in only one or two stores). However, since I buy virtually all of my clothing at the Gap and Old Navy, I decided it might be worth it to get one of their cards. To get the most out of the signup bonus of 15% off my first purchase, I waited until I needed to buy several expensive items and was able to save about $25 on that purchase alone.
I find this card to be particularly valuable because Gap sends out rewards certificates every time you spend a certain amount of money. The rewards are equivalent to 5% back, and I don't know of any other way to consistently get 5% back on my clothing purchases. The rewards are sent promptly with each credit card statement.
The card can also be used at Old Navy and Banana Republic, though I assume you can have all three store cards if you want to get three signup bonuses. Along with my initial signup bonus also came coupons for 10% off my next in-store purchase with the card and 10% off my first online purchase with the card (both deals were for each of the three stores). That's a lot of coupons.
There are also sales and coupon codes available throughout the year exclusively to cardholders, though many of them encourage what I consider to be high spending (for example, I recently received a coupon for $10 off a purchase of $75, but I don't often spend $75 on clothes all at once).
Normally, I just try to find coupon codes online, but Gap's coupon codes for cardholders don't work unless you pay with your card, and there don't seem to be many other coupon codes floating around out there. Some sites even say that Gap has specifically contacted them and asked them not to post and coupon codes not available to the general public.
Of course, right after I signed up for my card, Gap decided to change its terms so that cardholders no longer get free shipping for online orders at $50--I now have to spend $100 to get free shipping, a deal which used to be available to the general public. If I want to get free shipping on all my purchases, I have to spend $800 in a calendar year to achieve Gap Silver status and then spend $800 in each subsequent calendar year to maintain the free shipping.
If you want to try to reach that goal, sign up for the card in January to give yourself the most calendar days to reach the $800 threshold. But don't buy things you don't need just to get free shipping! That doesn't really save you any money. Or does it? Some might argue that, if done right, you're just converting shipping dollars to clothing dollars, which is a better use of your money, but you don't get free shipping on the first $800 you spend unless you purchase in $100 increments, and anyway, that's a math problem problem with too many variables that I don't feel like solving. (If you do, feel free to share your conclusions in the comments section.) For me, if my normal spending habits get me to $800, great, and if not, no biggie.