Not a bad game, but its EXTREMLY repeatative and gets rather tedious once you have everything, the bosses are just the same, different coloured, then upgraded stats, it's just your run-of-the-middle Zombie-shoot'em up.
The Combo system is glitched with the upgrades, I had 2x speed forever which annoyed me because it just ruined the fun...
Final aspect: Only play if you're bored, but meh...good guns, bad system, too easy, repatative...