At first I thought that this game was going to end up boring me and I would quit a few minutes into starting it, but I was delighted to be proven wrong. I think it was the slow start to the game which made me feel like it was boring, but it quickly picked up with the introduction of new enemies and the numerous modifications you made available for your mech.
Another thing which grabbed my interest was the change in music as soon as you got a bit farther into the game (I think around where the vehicles show up) which introduced not so much a change of pace, but it certainly felt like one with the quicker and more upbeat tunes.
Some things you could work on, however, are probably the locations. People might enjoy this a bit more if it was side scrolling/backgrounds changed with each level. It would give some people that sense of moving that some people find necessary to make a game enjoyable for them and feel like it isn't very repetitive. Other than that you did a great job, congrats.