I've been a little busy and we've gone right from one hurricane to another down here, so I haven't played the expansion yet. I started playing in May, just ahead of the CK release.
@Kalopsia predicted that I'd sour on the game and we wasn't totally off the mark. The repetition set in shortly thereafter, but I think the improvements made by Bethesda since release and over the summer have been pretty solid, addressing some of the bigger shortcomings. I still enjoy it, but that initial excitement has cooled significantly.
There's been a better brand of mods since the CK release, and pretty much everything has been ported to console. They did a content release that included the new buggy that you can add to your ship for pretty cheap, allowing you to drive around on planets instead of going on foot. Wasn't really my thing, but it is fun and well-implemented, and the audience really likes it.
Shattered Space, from what I hear, is a response to the criticism the game received at launch -- too much procgen, not enough deep, explorable content akin to Elder Scrolls and Fallout. So the expansion adds the serpent cult homeworld; sort Starfield's answer to being able to go to the Klingon or Romulan homeworlds.
The content on that planet is apparently much more hand-crafted with a bit of a horror vibe, and the response from the fanbase that stuck with the game was initially positive. But of course they also gobbled it up in record time, which led to a "Fun but MOAR NOW please" type of thing. Returning players and game reviewers had some good things to say, but mention that it's not a ton of content for a year+ of development and the minor pivots in content approach just emphasize how lacking the original game was. So the overall reviews have been mixed to negative.
If you've never played Starfield, my recommendation would be to hold off until the price drops on a main game purchase that includes the DLC. That might come sooner rather than later if the continued disappointing response rattles Bethesda's commitment to it. Not that getting it cheaper makes it a better game, but it's more time for more patches, more mods, and so on.
If you're expecting the deeply immersive experience of Fallout or Skyrim, you'll likely appreciate it for a little while but ultimately fizzle. If you're just looking for a fun game with lots of content and you appreciate the genre, it's definitely good but arguably not worth the current retail price for both Starfield and Shattered Space.