This is the sort of retro game that reminds us that the reason we remember old games so fondly is some of them were freaking fantastic. Great design, and avoids the artificial difficulty so many games seem to be moving towards. Honest to god tough, and enjoyable for it. This will probably make the favorites list once I get further in it. =P
This is pretty solid. I think it might be worth considering making the player's weapons do more damage, in exchange for maybe more enemies? It just feels so weak when a huge zepplin takes 3 rockets (and machine gun fire, and flame troopers) to take down what looks like a radio controlled helicopter. Final boss seems to put out way too much damage, considering there's no way to avoid most of its attacks.
Speed changes don't seem to work quite right; sometimes the slowdown will last long after you leave the grass, sometimes the grass takes a second to slow you down. Hitbox on the enemies is awfully big.
Could really stand to make the screen start scrolling earlier when you move towards one side. Also, it's annoying that you default back to the machine gun when you switch back to normal form. Make it remember your last weapon and go to that. Flaws aside though, this was pretty fun. Good job.
This was almost an instant 5/5 from me, but I had to make it a 4 at the end--it's a lot of fun, the art is great, just very well made in general. But it's missing a mute button, it could really use a save (just because of length, not because of difficulty), and getting Spirit Bombed 5 times in a row sort of put me off--that's not challenge, that's stalling.