It's really simple. What do you expect to spend at Gamestop over the next year?
Someone else mentioned $150, which is a rough estimate of what it takes to "recoup" the cost. It also depends on how much you value the magazine, which is okay to read on the shitter. There is also a buy 1, get 1 free coupon for signing up.
In other words, if you were already planning to buy three used games priced at $30, it doesn't make you worse off by purchasing it.
At the end of the day, the way I look at it is this. Don't get the card to do something you wouldn't already be doing. However, if you were already planning to do something, see how purchasing the card would fit in.
That's the key with these things. Don't let them influence you.