I'm really enjoying this, it's exactly what I hope for in a megawad. I've been really taking my time with it, but I'm up to E4M7 now and this is definitely going in my Replay folder. Also I think just playing through it has taught me some about mapping. Great work!
I did notice one minor issue that I haven't seen anyone mention yet: in E3M8 the cacodemon in the first large area can clip with the skybox if it goes in part of sector 106. Not a huge deal, but figured I'd mention it.