Pretty good...now make it better.
This game seems to have a pretty solid engine all the way around. played for a while had no glithces or "that doesnt make any sense" moments.
A graphics upgrade(not that they were bad, just could be an easy upgrade) as well as more unique characters coming at you. Not necessarily more types, just different skins. That alone would have given you another star at lest(depending on the chars, maybe 2). Such a small touch would add loads of playability to the game.
Could throw in more weapons, but there isnt any need to. More weapons could also cause more problems so i would do it only if YOU really like the idea and went easy on it.