Android Assault certainly doesn't rank among the all time scrolling shooters list, and that's not because of it's Sega CD format. It's a very solid game, and it controls well, but it just doesn't have enough uniqueness and style to be truly a classic.
It's not a bullet hell game, which in my book is a good thing. Collision detection is forgiving enough without being too easy. While the sprites aren't super exciting, there are lots of large non-boss sprites, and their animation is nice. Knock off half a point for parallax background scrolling that isn't always smooth.
I enjoyed changing directions and dodging landscape elements a la Vanguard, and I also very much like the CD music in the background. It's a nice touch that you do miss on otherwise fine cartridge shooters like one of my favorites, Gaiares. The loading times are speedy, I like that as well.
Learning the layout through repeated play is a hallmark of these games, and you're really going to learn this one. Enter a level with the wrong weapons transformation and you will definitely suffer. In this game I think that adds to the play, since you can get through with the wrong weapons. You'll just make a note if it for next time.
Another interesting aspect is that you charge up your weapon by not holding down any button. It feels a little weird, and honestly slows the pace a bit, but, well, sometimes a change of pace is a good thing. Your mileage with this feature may vary.
This is a game that will definitely see more play time on my Sega CD. It's nice when you want a scrolling shooter but not a crazy fast pace. This one's a little more brainy than brawny.