In PSI, the dungeons have that 3D feel to them as you navigate the passages. Have you ever resorted to using maps or drawing them yourself, or did you go off memory? Heck, I am certain there's people who've just taken wild guesses. xD
As for myself, I'd memorize the maps. The only things that got me were the pits where you fell down, but other than that...
As for PSII-IV... Unlike PSI, they were completely 2D, so in my opinion there's no comparison in the difficulty since you have a general idea of where you are going at all times. ^_^ So did anyone have a hard time with those?
Admittedly, a few of the maps (chiefly PSII) were THAT difficult...namely one comes to mind, Ikuto on Dezo (the palace that was hidden until you received the aero-prism, it had the dungeon where you fell down holes...many many holes ). That was the ONLY dungeon I had to look up. >_<
Also, if there is a thread like this already, then I was unaware. =D