It feels very incomplete, and here's why:
When you shoot things that take more than one shot to kill, there's no indication that anything has happened. A smaller explosion or even a white flash of the enemy accompanied by a sound effect will make things seem more dynamic and less tedious, like when Sonic the Hedgehog jumps on Dr. Robotnik and his machine flashes and there's that sound like breaking blocks. It might not seem like much, but getting a response out of the game is important.
I think it'd be cool if the ship tilted or something when it went from side to side, but I realize that that might not be the kind of ship you had in mind. Still, a little animation on the ship would make the game seem more immersive.
All in all, there's nothing about this game that really makes it stand out from the crowd in a positive light. Also, I just read that there are selectable characters *goes back to check* Ooo, it's cool that you included more maneuverable ships: to begin with I just figured you'd be able to upgrade the maneuverability of Dalton's ship. ^.^;; But, man...the girl's ship is incredibly weak.
I hope you keep these suggestions in mind if you make another game like this: I think they'd help everything feel more action-packed.