We all know the reason they are not great: there's so mega much to read!
A well-written megadungeon doesn't require you to read it all, however. You should be able to run your first session by just reading about the area around the main entrance to the first level. Everything else should come later.
Do not waste time role-playing in the nearby town before adventuring in the dungeon.
Do not waste time exploring the wilderness around the main entrance before adventuring in the dungeon.
Do not waste time searching for other entries before using the main entrance.
A dungeon has "regions" that are not easily traversed because there aren't all that many places to cross from one region to the next. That's why you can get away with just preparing the area around the main entrance to the first level.
The benefit of this approach is that players will revisit the area around the main entrance to the first level many times. And this is true all throughout the megadungeon. Players will return to known locations many times. For the person running the megadungeon, this is great. Only small things have changed as far as prep is concerned but the interactions between the things is vastly different.
The map remains the same. The rooms remain the same. Perhaps new monsters have shown up. Perhaps this time they have a different routine. Perhaps they don't want to fight, this time. Perhaps they're complaining about something and need the party's help, this time.
This is the thing! This is why megadungeons are great: The locations, numbers, attitudes and reactions of dungeon denizens is easy to change on the spot but writing and drawing a new dungeon is not. Every location can be visited many times and it'll still be interesting. New traps, new monsters, new treasures; same monsters, new reactions, new quests, new allies. This stuff is easy for us to improvise once we have a map and the initial setup.
#RPG #Megadungeon