Alien Syndrome was first an arcade "run-and-gun" game in 1987, then a Master System port in 1988, then a port to every system on the planet, and later (1992) a Game Gear sequel with the same name. There was another sequel in 2007 but we don't care about that. Here I shall talk about the two Sega 8-bit home console versions, of course.
Graphics
The game has a zero-degrees oblique projection, which is a kind of crappy 3D where you can't really see the floor a lot of the time. On the SMS the graphics are 1988 first-party sparse blobs of colour, while the GG is more 1992 second-party effort that looks like an actual pixel artist was involved.
Sprites are similarly uninspired. The GG ones are again a bit better drawn.
The GG version's title screen has a neat graphical stretching effect, which is actually really hard to do on the Sega 8-bit systems.
Sound
I played with it off.
Longevity
The levels tend to get more difficult to traverse in the time given, plus the later stages tend to have more difficult enemies. That's where the difficulty curve is supposed to come from.
On the SMS, the game screen-flips as you walk around the environment. This is mainly to allow it to have an RPG-style system where enemies randomly spawn on each screen without the game having to track any off-screen enemies, but it also means you can cheat RPG-style by screen-flipping back and forth to reset the enemies.
The GG version instead has full-level scrolling, and much fewer enemies as a result; but the small screen means you don't notice that too much. It also means enemies can follow you around the world (although off-screen they seem not to do anything), meaning you can see the quality of 1992 8-bit route finding. It also gives you an in-game map, making it a lot easier to find your way around and to plan your route, which cuts out a lot of the need to replay levels.
So, things are made easier by technical shortcomings and design decisions of the games' implementations.
I was generally shocked by how short the GG version is, and the SMS version is likewise not too long - although it is about the length you'd expect for an arcade game. However, on both - but to a much greater extent on the SMS - the boss battles are harder than the main stages. The final boss on SMS seemed stupidly hard to defeat, although it seems there are some tricks in the game to make it easier.
Plot
SMS: In 2089, aliens invade human-occupied space. The foolish humans fight back at their alien overlords and are taken hostage. Only two members of the Earth Command Troopers volunteer to go on a rescue mission - Ricky and Mary. No, sorry: they're RICKY and MARY, apparently.
GG: In 2000, aliens invaded Space Colony "Alpha" and took the crew hostage. RICKY and MARY, members of "SCOT", rescued them. Now it's 5 years later and it's happened again.
(Note that one of the GG variants has amusing Engrish, including the infamous "emergency massage" that gave the Massage emulator its name.)
Fun
Running and gunning is fun in itself, but this is a simplistic and flawed implementation. The crappy 3D makes it hard to see what's going on. The poor graphics, especially the player sprites, make it not particularly nice to look at. The wacky weapon upgrade system, especially when you can't know which weapon suits the next boss, makes it rather infuriating. The bosses are all rather non-sequitur. The overall feeling is that it could be a lot more fun.
Innovation
I'm not sure how innovative the original arcade game was, but the console versions seem very by-the-numbers. The later Game Gear release gets less leeway as we really knew better by then. There could have been a lot more gun types (the GG version at least has upgrades, I think), more power-ups, more enemy types, enemy attack formations, all the stuff shoot-em-ups had been incrementally developing for some time.
Conclusion
Not really a classic, this is one of the many "arcade hits" that filled the Sega first-party catalogue but hasn't really withstood the ravages of time. The GG sequel (presuming they meant to say 2089 in the intro) manages to improve on a few aspects and then screws it up by being easier and shorter.
