Having been in the alpha/beta of this game and having recently purchased (finally) Borderlands 2 and all of its content... there's no way in hell I'd ever pay cash money for Defiance, even if I am enjoying the show. Some of the game is fun, but after playing the two, BL2 is just so much MORE fun. The variety of weapons, the crazy stuff they can do, how enemies react when you shoot them, etc. That's not even going into the comedy of BL2 versus fairly bog standard and boring story presented in Defiance.
I also feel like they missed the mark not placing the game where the action is taking place in the story. You would have more opportunity to interact with the main characters but the most important thing is that they could have expanded upon the underground St. Louis thing more and made it a massive sprawling post-apoc underground city dungeon... that could have been amazing.
Sure... Defiance has Rifts but if I want those types of dynamic events I can go play... Rift... or even Guild Wars 2.
The more I get into the show and the more I play Borderlands 2, the more I see massive wasted potential in Defiance the Game.