It was intresting.
The main boss was a bit of a dissapointment, and even after getting all the trophies, it only took me 15 minutes to beat it. If you make a sequal, try and make the sonar used in the boss fights, such as shooting the boss enough causing its jaw to go slack, letting you fly into it to do real damage, but you have to get out in X amount of time or your trapped. Maybe have it highlight weak points, or even be uber evil and make it the only way to see the boss at all, as it moves around.
It was an intresting idea, and unique, as far as any game I have ever played. It still could have been better, maybe with the standard enimies chasing or attacking you, with or with out the gun as a power up. Maybe spinning levels or floating debris' that move around in a certain pattern? Just tossing out ideas. I would love to see an expanded sequal with more features.