A couple years back, inspired by Dragonsfoot and the OD&D Discussion Forum, I started working on my own megadungeon. At the time, I think I had too many ideas preconceived about what the place should be like. In the end, I ended up not too happy with several of the level maps.
I realize now, looking at them, what mistakes (at least from my own perspective) I made. I wanted themed zone areas, but basically gave them all choke point entrances. The first level is sort of the epitome of it. I created an entrance room with a corridor going in each of the 4 cardinal directions, but those corridors each led to a discrete segment of the first level. Yes, there were multiple access points to the second or lower levels from there, but having the first level be so confined really cramped the style.
It also wasn't really big enough. This is also a problem with many of the lower levels, if you assume you should adventure on a level multiple times until you've gone up one level, then venture deeper, and you still shouldn't have completed everything on the upper level.
My old Yamanashi group did, in a couple of marathon sessions, explore quite a bit of that first level, and a fraction of the second. We had a lot of fun, but I wasn't satisfied with the layout from a DM perspective. I don't think the players really had enough time to get a feel for it, but my not being happy with it was enough. I did retroactively add a few connections between the segments (on the scan above), but it was too little, too late.
So I redrew the first level. I still haven't stocked it yet. For one thing, my current group isn't big on the megadungeon idea. For another, I plan to lift quite a few of the encounters from the original, but it seems like a bit of a pain to figure out where they should go without a group actively exploring it. I figure I'll get around to it some day. Maybe in a couple years, when my son is old enough to play. Here's hoping!