If you already know and love the cobalt strings, stop reading and buy this. If you need some convincing - cobalt strings have a slight improvement in tone and general sound that you need to experience to understand. As a guy who tends to sweat a lot, I also found they last longer than regular strings. The M-steel is even a little bit better than cobalts.
You get 2 sets of awesome clear strings, and the way I see it, an extra backup set for emergencies. Great deal and definitely worth it's price!