Nice game! Very smooth graphics, explosions, and sound effects.
Some suggestions for improvements (but I suck at games sometimes so take it for what it is worth):
1. I found it too hard. On average, I'm only able to last 1-2 min.
2. It was too monotonous even though it was very short play. I just hold down the W & E keys and swing back and forth trying to shoot down green ships. There is little chance for strategy and all the explosions are mind numbing (can't see where enemies are) after awhile.
3. A game without strategy isn't bad, it is just that I think some variety is needed and a chance to use more skill in shooting. As of right now, it is pretty much try to get in front of the green ships without getting hit by bullets from the larger ships.
4. I wish I could move forward and backwards, because the enemy will always kill you once it reaches the end of the screen. Or instead give a temporary bomb/force-field capability.
5. Needs additional types of enemies.
6. Add some powerups to give player more capability.
Still really nice work for a beta.