Here are my opinions...
The gameplay is a lot of fun. It's kind of a throwback to the old 1979 arcade game, Asteroids. One thing I didn't like is that the power-ups end after a certain amount of time. I would like it better if I could keep the power-ups until I got hit by an asteroid. This is especially true in the case of the shield.
I also wasn't a big fan of the controls, my thoughts on each control scheme below...
Joystick:
This is the control setting that I tried first, and I immediately didn't like it. I don't like that I couldn't turn without also moving forward. I want my turning and thrust to be seperate. While playing with this control scheme I was yearning for dual joystick controls (where the left stick controls your movement and the right stick idependently controls aiming and shooting). But dual sticks would change the gameplay because you could aim and shoot in a different direction than you are moving, and I don't know if you intend that change. (Althought it's a change that I would welcome.)
Tilt:
The tilt control was the second control setting I tried. I like it better than the Joystick control, but I didn't like that there is no option to calibrate the tilt settings. Usually when playing a game I'm holding the phone at an angle. I find it very akward to hold the phone perfectly flat and look down on it from above. Other games that use tilt controls usually have an option to calibrate the controls that way you can pick a comfortable position for you and calibrate that to the neutral position.
Buttons
Finally I tried the button control scheme and I like it the best because it allowed me to seperate my motion from aiming. I could thrust forward, stop thrust, and then aim in any direction while still moving forward. It still suffered from the same need of a tilt calibration.
Overall, this is a fun game. I don't think I'd pay for it, but for a free game it's pretty entertaining. It has a lot of potential, but it doesn't feel quite ready to me. If you make some changes to the controls and allow the powerups to stay until you get hit, you'll have a winner on your hands.
I hope this helps.