I quite liked the 1e Manual of the Planes. It described what each plane looked like, how to travel between them, etc. Acheron, in particular, was interesting--the land consisted of floating blocks. The deepest level consisted of floating, moving planes, so razor thin that there were rules for being cut by one as it passed.
I never saw anything from Planescape, so I never used any of that information. It's probably all built off the 1e info.
It's the same in Planescape. Layer one is giant metal cubes. Layer two is more cubes, but the layer also turns anything there into stone after a while. Layer three is polyhedra of all sorts, as if the Far Realm dumped a stupendously gargantuan Crown Royal dice bag all open all over the layer.

Layer four is the nasty shards of ice that can slice you in half, some miles long, some so small they can't be seen. I'm pretty sure Planescape took all the old 1e material and just added even more stuff to it. And 3e basically summed up that material in MotP.
I'm surprised Acheron is so popular. It's an interesting place to send a party (like all the lower planes

) but I really can't figure out how to fit something like that into my cosmology. I think the real problem is that it doesn't have it's own race of outsiders, except the bladelings, but they're not really that significant.
I think maybe that's the problem with the buffer planes. They don't have their own major planar races, which makes them feel less important. Celestia had/had the archons, Elysium has the guardinals, Arborea has the eladrin, Mechanus has the modrons, the Outlands have the rilmani, Limbo has the slaadi, the Nine Hells have the devils, and the Abyss has the demons. The yugoloths use both Gehenna and the Gray Waste. There are the gehreleths on Carceri, but they're nothing in the grand scheme of planar affairs.
The buffer planes sometimes are home to various pantheons, so you've got orcs and goblins on Acheron, gnomes on Bytopia, and the Norse gods on Ysgard. And I'm more interested in a cosmology where the gods' realms are seperate planes away from the rest of the outer planes themselves.
Or maybe it's that some planes for me just don't have as strong a hook. That's why I'm thinking of just dropping them in a more customized cosmology. That's the case with Arcadia, Bytopia, and Acheron, they just don't have enough for me to put them into my campaign. Celestia works pretty well as the main good outer plane, and the same for the Nine Hells and the Abyss for evil. Bytopia and Elysium feel redundant, and while I like some of the aspects of the guardinals, good outsiders are already pretty well covered by angels, archons, and eladrin. Ysgard has some cool stuff, but I'd probably just use the top layer as a divine realm of a war god. Pandemonium is also a plane I like, but the layers are similar enough that I could probably just use it as a single Abyssal layer, using the wind-filled caverns as the main layer, while the bottom layer of Pandemonium is on the outer edge of the network of caverns, or maybe there's bubbles and pockets in between the caverns. The lower Gray Waste would be turned into the realm of the god of death, while I'd combine Oinos and Gehenna into one plane: have the volcanoes floating above a bleak and blasted disease-ridden landscape where the armies of the devils and demons perpetually fight the futile battles of the Blood War, while the yugoloths watch over them from their citadels above.