What a fantastic game! It is a technical masterpiece for Sega's 16 bit hardware capabilities. This game contains huge enemies that consist of many sprites and pulls off some impressive feats. Some of the feats include scaling, rotation, and there is even a butterfly boss that uses actual polygons. Many people thought the Genesis could not do these abilities. But it certainly can and the proof of that is Alien Soldier. Plus, the pixel art is done so well and detailed that it makes it, in my opinion, one of the best looking 16-bit games. Not bad for a console that can only output 64 colors in gameplay use.
Alien Soldier is basically a boss rush game where you fight boss after boss. The game is hard, but rewards you every time you advance to the next stage. My advice is that you play on supereasy first, not superhard. You can change this in the options menu before starting the game. Although, if you want to prove your manlihood (or bird manlihood?)go for superhard.
Vastly underrated, you must play it! The Japanese and European cartridges are expensive, but the repros are not. Get them!