I picked up Alien Syndrome on a whim the other day -- the first one I've picked up since Space Harrier. Just some quick impressions.
I was a huge fan of the original in the coin-op. Poured tons of quarters into it. It definitely holds a place for me in my gaming as a youngster. This remake, while nice, defintiely has some issues.
-They've made the game much darker (literally). You can't even see the walls in some stages.
-As w/ other Sega Ages, the models are kinda cheesy
-Bosses are re-arranged, and in some cases, very different
-Game is harder, but now has a life meter
I liked the original, in that you could continue up to the last stage. It at least required some skill. This time, they made the last boss so damn hard (even on easy) I doubt you'd kill him w/o tons of practice.
They have kept it a 2D style game -- IE, you can't move the camera around, and the stage layout appears to be the same, from what I remember.
It was a cheapy, so even if it sucked, it's no big deal. I'd asy it's a fair game, and worth picking up if you liked the original.