Undoubtedly the best scrolling shooter there was, and still is now.
Multiple play modes including 1 player arcade, full story mode, 2 player cooperative and there used to be a network mode cooperative but I'm not sure if they still have this (it was on the shareware demo, at least).
Kill enemies to get points, and buy better weapons (or upgrade your weapon up to 10 times) with those points.
Has a very good soundtrack, and nice graphics for its time.
I'd still rate it highly.
I think it's almost impossible to play without the DOS-Box emulator.

Yeah, it looks good, haven't played it. There's a lot like it, though, for example, Demon Star (though that doesn't have upgrades, I think).