Great game mechanics, but it limits the amount you can use them.
The weak story should be the biggest problem this game has, and the speed and shooting mechanics should be it's greatest strengths. Unfortunately the game suffers from what a lot of it's predecessors also suffered from; the gimmick that makes the game is only given out in small doses.
If I could move through every board at high speed learning to fire in a new way it would have been great, but unfortunately all the abilities are tied to the same bar. Which means that high speed movement is tied to health, and armor... and melee. If you get hit then you can't move at high speed. If you move at high speed for too long then you have no armor, if you hit melee then you can't run away or defend yourself.
If these abilities were freely given the game would have been much better, but by limiting them, the game is held back.
Also, the weapon upgrade system is based upon getting more of the same gun and increasing it, doing so makes it undesirable to try out different weapons because they'll just be weaker versions.
hopefully a sequel will come out, hopefully it will fully embrace the ideas put in the first one, but as for this version $20 is more than you should pay.