Brutal! The starting difficulty was way too high for me! :( I probably died 10-15 times before I even figured out how the basic controls work.
I do like the central concept that the tank faces and shoots in only one direction, and that direction changes after each enemy you shoot. That's pretty clever and did keep me on my toes once I figured it out.
My advice would be to slow the enemies way down and then slowly ramp up their speed up, speeding them up just a little bit after each enemy is shot.