Best Buy has just confirmed on their Facebook page that this coupon offer is legit!
Save $50 on a $100+ purchase in stores only when you bring this coupon and use your Mastercard!
The exclusion list is MASSIVE. Here are some ideas for non-excluded items: entertainment media (CD's, DVDs, Video Games), non-Samsung or Sony TVs, most computer accessories, memory, etc. See the coupon for all of the exclusions. Reports are that they are updating with more exclusions though.
Don't have a Mastercard? Buy a Mastercard gift card. It doesn't say any specific type of Mastercard, so there should not be an issue with that.