I actually just finished this quite recently, playing a PSOne copy on my PS3 (but hey man, backward compatibility is stupid, right?) and it holds up. The graphics are grainy, but they were at the time, too. I remember reading PSM gushing about the visuals, and they were nice, but never did I look at the characters and freak out or anything. The environments and machines stand out as quite nice, though.
The controls, specifically aiming the pistol and machine gun, feel weirder now than they did in '98. Aw well, I got through the game without much problem.
Because I suspect someone will shout about it: yes, I'm aware of all the differences in The Twin Snakes. It's a joke. 😉