This deal is definitely good if you need to buy anything at Staples (who price matches other stores), but might also be good for use elsewhere. Read more ...
HERE is the deal, good through tomorrow (or while supplies last). You will know that you are getting the $50 promo card if it shows up on the checkout page on the website (on the right side of the screen). Obviously, this is only a good deal because of the $50 promo card. And note that the promo card is only available for use during the month of December, whereas the $300 gift card is good right away and can be used anytime.
This is obviously a good deal if you're looking to buy anything at Staples. And note that Staples will price match most major retailers (including Amazon).
What makes this a particularly interesting deal is that you can maybe buy other retailers gift cards (such as Sears, Amazon, and many others) in a Staples store (not on Staples.com) with these gift cards (both the $300 one and the $50 promo one). Full disclosure: the fine print on the $300 one (not the $50 one) says that it is not good for buying other retailers gift cards, but I had no problem using them to buy gift cards. But you need to be prepared for it not working. So if you don't care about buying stuff at Staples, you might be "stuck" needing to resell the Staples gift cards (but you should still be able to get at least $300 or so, maybe a bit more, by reselling the $350 in gift cards).