Hmmm. Not bad.
.....at times, I actually think this game should be harder, here's why:
If you play your cards right, and exploit the respawn of the two flying robots in the L1 boss battle, you can have every single upgrade before L2. And if you've got that, you're pretty much sorted, so long as you grind a little more so you can buy some nice ammo.
I'd quite like (if there's ever another game like this) less grinding potential, and a bit more cash in the first levels.......and perhaps more variety in the background.
Apart from that, it's a nice little time-killer that I keep coming back to.