Was a bit hesitant to buy this since the Guardians of the Galaxy aren't that appealing to me (even though I liked the film, never was a huge fan of the comic) but it adds some interesting elements to the game. There are counters now called Shards that allows you to add power to your attack every time you gain one creating some massive attacks later on in the game if you stock pile some, which you'll probably need against Thanos. I did like the Kree and the Supreme Intelligence being added to the game.
It's a good buy, better than Paint the Town Red, but obviously not as good as Dark City. I'd recommend it.