One of the cool things about the game other than the license (which is, let's be honest, the best thing it has going for it) is that there's a protracted intro sequence before the Things start showing up. So few games do this - suspensefully set up a dangerous situation before thrusting the player into a gunfight - that I remember it being notable for the first hour or so alone. Half Lives 1 & 2 did this pretty well, too.
I haven't played it since it first came out on the PS2, so I can't vouch for how well things have held up. It probably looks pretty terrible by now.