It's been said many times before....this game is extremely slow. The scrolling, the ship movements, your projectiles, enemy projectiles, everything moves at a snail's pace. Shooters are supposed to be fast and intense....of which this game is neither.
You're also fighting over the same terrain in every level....Mars isnt all just a barren red dusty plain. There are canyons, mountains, volcanoes, polar ice caps, etc... It would have been cool to see some of that variety in the scrolling scenery.
The bosses were ok....but they lacked variety. They all had roughly the same fighting pattern: dodge a round of normal shots followed by two missiles. Each boss needs to have some unique attacks and abilities that sets it apart from all the other bosses. Most shooters also have some kind of indicator as to what the boss's current health is...another thing this game doesn't have. Some multi-stage bosses would also have made the boss fights more interesting.
If there was anything in this game that disappointed me the most, it would have to be the complete ineffectiveness of your ship's weapons, especially in the later levels. Even on the highest upgrades, it takes about 20 to 30 seconds just to kill a single enemy ship...and it's not even a boss. Most of the time I barely kill just one out of 3 or 4 enemy ships on the screen before they all fly off. The weapon upgrades were sad too, doing nothing but increasing the number and fire-rate (barely) of your shots. The amount of damage done by each shot never changes. At the max upgrade (I assumed since I saw no other visible changes afterwards), you start shooting 2 missiles, but they are dumb as bricks, often flying off the screen and circling into nothingness instead of homing onto a nearby enemy.