Well presented, but completely irritating.
HP of enemies increases suddenly after the first dozen so you need a power-up which you may or may not get, especially irritating when it's near the edge of the screen where you can't pick it up. After leveling-up a weapon, picking up a power-up at the wrong time leaves you with a totally useless weapon.
Enemy shots would be better a bit larger to make 'em more visible
The aim is to make it fun for the player, not to kill 'em off ASAP. Take a look at Zenon Mega Blast, StarMada or Frantic for how it should be done.