Spelljammer was released and initially served as a bridge between the various extant settings of Dungeons & Dragons. The Forgotten Realms was in its own Crystal Sphere but you could travel over to Greyhawk if you wanted. In between lay Horatio Hornblower in Space.
But Spelljammer lacked much in the way of a setting of its own. There are some implied elements like the Unhuman War and the Imperial Elven Navy and a couple of more detailed setpieces like the Rock of Bral but that's one city. Enter The Astromundi Cluster.
In conceiving of a Crystal Sphere for Spelljammer what's the first thing a designer out to do? Get rid of the planets. Shattered Space has no groundlings because the planets in the sphere were destroyed in great celestial cataclysms. Civilization is spread out across multiple asteroids and other such bodies. Multiple factions vie for control of Shattered Space including Illithids, Neogi, and several different stripes of Human. Almost every major faction has a plan that would, accidentally or intentionally, basically kill everything in the Sphere. So, you can pick one plan to explore in a campaign (and the two most prominent are well detailed) or, if you're a maniac, you could run all of the intrigues simultaneously!
To give Spelljammer its own two feet to stand on, a lot of changes have been implemented. The Mind Flayers are now mutant descendants of humans who were forced below ground for generations. Not using the stock Illithid D&D had thus far used is a good way to make them a bit fresher. Their personalities are largely unchanged, just their origin has been adjusted to something newer. The Neogi are now not so much moustache-twirling, slaving, cannibals as they are arch-capitalists. I mean, they still do the moustache-twirling, slavery, and cannibalism. But they have slightly more going on than being so evil their ships are shoot-on-sight. In fact, being the center of trade means the Neogi are the biggest factor keeping the peace in the Sphere. If anyone ramps up conflict too much the Neogi bring down a trade embargo, crippling them.
Much like the Legend of Spelljammer box set, the setting here teeters on a knife's edge. Every faction is seeking an edge over every other faction and a little push (like say, from a group of adventurers) could severely tip the scales. It's a setting rife with conflict and is so precariously balanced that a party of PCs could easily be a driving force in the plot. There are some interesting ideas in the dozens of quest hooks the books supply and while the Astromundi Cluster might be fun to dip into, I don't know that I'd want to spend a whole campaign there.
But it's a solid attempt to give Spelljammer something big to call its own.
I don't generally buy campaign setting books, but since this was part of the SpellJammer line I had to pick it up. My single biggest problem with the Astromundi cluster was that it was too contained, that it allowed no real chance to escape into the wider galaxy of spelljamming. I know that there are a lot of people who hated the crossover setting approach that Spelljammer took, I personally hated having to deal with the Dragonlance steel for gold exchange scheme that by players came up with, but there was no reason to trap players in a single sphere for an entire campaign.