Well I know, because I wrote (approximately) half of it. :) It was written well in that it was a technically innovative and beautiful game that pushed the boundaries of the PS1 hardware. It was written well in that the shipped code didn't crash or have horrible show-stopping problems. (This was important because there were no streamed patches back in the days of physical media.)

But it was absolutely not written well from the standpoint of maintainability, abstraction, documentation, testability, etc. This is partly due to the insane time constraints we were under, partly due to the primitive nature of the development tools at the time, and partly due to the intrinsically low-abstraction methods required to achieve the necessary performance -- e.g., the entire renderer and collision detection system were written in MIPS assembly (by me).