I'm having a love hate relationship with the U6P dungeons. If nothing else, you have to marvel at the level of complexity that went into designing them. They truly are massive and you can spend hours upon hours just wandering around.
However, due to the limited availability of in game maps for the subteranean areas (they all exist, but finding them is damn near impossible some times), the inability (not sure if it's purposeful or a limit of the DS engine) to see you location on those maps, and no 'fog of war' on the maps, it makes navigating them, when you need to find something specific, very, very challenging.
I tried drawing some of my own maps (like the old days), but it's pretty tough given the detail and size of them. I found myself completely lost in the sewers more than once and had to teleport back to the throne room and try again. Due to the proximity of the throne room to the sewers, that's not a bad solution, but that doesn't work on most of the dungeons that aren't located right near shrines or moongates.
One thing I did to make my life a little easier down there, which some probably consider a cheat, is to crank up the gamma correction on the display settings. It washes out the shadows and such a lot, but it makes it a hell of a lot easier to navigate around.
Despite all my complaining though, I'm totally loving it!