Played five minutes. First impressions:
Energy drains at a constant rate while firing, which feels weird but isn't really noticeable. I think it should drain a set amount per shot fired.
The penalty for running out of energy is just a tad too long, IMO.
Unwarranted suggestions:
Energy meters floating on the UI near your ship would be useful, instead of having to look in the corner.
Would be cool if different types of weapons had gravity. A physical bullet based gun could wrap around the planet a bit, for example.