A store credit card may not be worth the 20% off that they offer you in order to entice you into getting the card and in the long run it may end up costing you much more in terms of a lowered credit score, higher interest rates and in the case of ?pay no interest? ending up having to pay all of the interest and more if you miss the cut-off by just a day or two.
Check out the dangers of private-label cards or store brand cards here.