The graphics are nice, but get muddy in later levels, and enemies are hard to spot. The enemy movements are enjoyable and challenging, but become unfair in level 4.
The weapon upgrade system is pointless, since every weapon seems to have the same fire power.
The scoring system is useless. Harder enemies don't get you more points than easy enemies. The further you progress the less points you collect, which is the opposite of what it should be.
Level 4 introduces obstacles on screen you can't touch, but still uses copies of the same enemy formations as before, and the 2 aren't compatible. It's sometimes simply impossible to shoot all the enemies in time to stop them from firing. Even if you have upgraded weapons, they can't shoot "through" enemy formations to get rid of more than one per shot, so the upgraded weapons seem pointless, since every enemy still needs exactly one shot. So it happens way too often that you have no room to manoeuvre and loads of bullets flying towards you, while your own bullets don't even seem to make a dent.
The obstacles in the level design in level 4 will absorb both "bullets" your ship shoots simultaneously, even if just one of them has the obstacle in the way, so it becomes even harder to find corners to get rid of stationary enemies.
All in all I do like motivational challenges, but sloppy level design, and a game engine not fit for the levels create challenges that don't motivate but annoy.