Actually, pretty good. Sure felt like PiD (aside from the lack of talking dead guys and some other minute details). Oh, and that M-79 is nasty: "Oh look, some Headless. Now let's see what this-- BOOM! Ugh, there goes my triple health." ;)
An incomplete scenario that was the prequel to Marathon: Dave (mainly known as Dave demo).
Please read the text document that comes with.