Well, so far, from the very limited anecdotes in this thread, those who actually played it and used it, enjoyed it.
Has anyone run this and not enjoyed it? That, to me, would be the strongest condemnation. "It doesn't have X" is not a condemnation. If you want X, add it. It's not like it's all that hard. You can use the published 2e stuff, outside of mechanics, as is. You don't even have to do conversions.
Did pick up a really cool random world generator from Reddit. Will have to go poke around my hard drive when I get home and I'll post it here.
My 2 cents:
I wasn't born when the original Spelljammer came out, but I read the 2E books a few years ago. My current campaign has a heavily-homebrewed version of the Spelljammer cosmology, where I combined it with Magic the Gathering-type planeswalkers.
I was very interested in the book before the came out. Then I read it. There are some useful rules stuff, but most of them give very vague ideas (how big air envelopes are etc.) that 1) already existed in the 2E books, sometimes with the same wording, 2) I could've come up with them using common sense. They completely axed any complex rules about space travel, and ships have a warp speed that is the same for all ships (so ship quality never affects travel times) and a regular speed -
about which there are no rules.
Seriously, the ship stat blocks are so barebones, and more importantly,
the ships in Spelljammer are not compatible with previous ship rules. I expected them to use the seafaring rules in Ghosts of Saltmarsh as a base, but ideas in GoS are abandoned. No other ship combat rules are used. It was so bad that I needed to get my hands on homebrew ship combat rules to have any use of the ship combat side of a
setting about spaceships (I got the Captains & Cannons supplement, in case anyone else would be interested in such rules).
Worldbuilding-wise, I didn't use anything from the book since the ideas in Xaryxis or the 5E Rock of Bral did not interest me, but that's on me. Nevertheless, there is clearly far less lore than earlier books and even similar 5E books. We have very few wildspace system descriptions, while the 2E book had rules on designing an entire solar system from scratch. As others said, there's no mention of how religion works between different wildspaces, even though that could be an interesting conceptual space.
As for the monsters, I think the list is SHOCKINGLY small. More importantly, WotC's decision to stop making content for high-level games really hurt the book's usefulness to me. My party was level 13-14 around the time they went to space, with a lot of powerful magic items and NPC sidekicks with them. As a result, I need a lot of high CR monsters to challenge them. Most of the monsters are in CR 3-9 range, so they weren't that useful to me, and the ones that I did use actually came from the D&D Beyond Web Supplement and not the book itself.
All in all, I ended up not using the book as much as I wanted. I needed to homebrew 90% of the Spelljammer content for my game, and the remaining 10% I probably could've gotten from delving deeper into the 2E books.