A really outstanding game. Good graphics, nice action and a great blend of genres. Sure its a bit repetitive at times, but I still find myself playing this game more than once per day.
Suggestions for improvement?
1. More powerups. I enjoyed the Nuke and Spike powerups, but perhaps something like an orbiting ship powerup, or a missile launcher powerup. Would be nice.
That's all I really can think of. Its a great game. A masterpiece.