If there's one word that can describe this game: "mindless". If I could use two, it would be "mindless distraction".
I barely remember the plot at all, and the parts that I did gather did not make a whole lot of sense.
Gameplay wise the most complicated "tactic" is to blow up barrels. All upgrades are simply improvements over the previous firearm/armor. There's no thinking required.
But if you love holding down the mouse key and blowing up swarms of recolored enemies, then this game might be for you. It's sort of theraputic actually.
Worth it if you can get it on sale. I figure it's worth about $3 of fun. Really I'm only writing this review because of a personal mission to review every game I own on Steam.