Having spent a bit of time, now, getting into Brilliant Diamond, I can say that my overall thought is that the game is... okay.
I genuinely don't have issues with these games. They're fine for what they are. The art style took a fair bit of getting used to, but, after spending some time with it, I barely notice it. I don't particularly like it, but I don't dislike it. There's a... blurring effect on the edge of the screen, at least sometimes. I've noticed it at Lake Verity and the Eterna Forest. I'm not entirely sure if it's location specific or not, but that's the only complaint I have with the art/graphics.
One thing that I've genuinely liked is the changes to the Underground. They're not extensive, and it's still primarily a little diversion, but I do appreciate the inclusion of the caves where you can encounter wild Pokémon, particularly Pokémon you wouldn't otherwise be able to encounter until later in the game. For the folks who are returning to these games after having played the original, that small change adds the hook that the games need to at least keep your attention. It's nice to be able to get, say, Skorupi or Murkrow before the second gym without needing to resort to trading.
Perhaps I'm biased in that regard, though, because I tend to think a lot about the folks - well, really, the kids - playing these games without access to Wi-Fi. It's certainly much rarer in this day and age, but there are still enough communities out there with spotty Internet infrastructure that I dislike games built on the assumption everyone has access to a strong, stable Internet connection and which lock content behind it, and I appreciate games that add more options and flexibility for folks without requiring an Internet connection.
That's a tangent, though. On the whole, as Brilliant Diamond and Shining Pearl, these games are fine. In a vacuum, they're a fun experience that holds up without much in the way of glaring flaws. I wouldn't wish them into un-existence or anything. I might even buy a copy for my niece and nephews, since they explicitly want more Switch games because they seem to prefer it to to the DS. I'm glad that these games exist for the folks who are playing them and experiencing Sinnoh for the first time (the QoL changes that do exist, like the removal of HMs and the new EXP Share and the "boxes from anywhere" system are nice for those folks) or who strongly wish to re-experience Sinnoh today, and I don't have any major issues with them on their own.
As remakes, however, I'll note that they are extremely disappointing. I'll at least credit ICLA and TPC and whoever else with not advertising these are more than they are, and presenting them clearly as faithful remakes, but that doesn't change the fact that they could be much more than they are. I would certainly have liked to see more done with this game. Reimagining of areas and expansion of the stories, as happened with the Sea Mauville-New Mauville-Mauville City and the many characters associated with them in ORAS, an overhaul of Team Galactic to make them more interesting like Aqua and Magma got in ORAS, potentially expansion of post-game content like FRLG had with the Sevii Islands, all of that could have and should have been a part of these games. Gamefreak - though they may be less directly involved on this game than the others - have repeatedly stated, in the context of discussing remakes - the challenge of balancing preserving people's memories and nostalgia for old games while also offering something fresh and new. It's not as though they or their partners are unaware that there's merit and value in creating a new experience, expanding on and reimagining what has been done before, keeping its core, but adding more heart and soul and making something fresh, so it's a shame to see the Diamond and Pearl remakes be less than all they could be.
Brilliant Diamond and Shining Pearl are fine. The Diamond and Pearl remakes are not, They're much less than they could have been. We know that this is a series that can innovate, that can build new experiences that incorporate and incentivize exploration and the creation of those moments of "wow! My persistence paid off! I've discovered this weird underwater area with a braille puzzle that, oh, is kicking off an entire chain of sidequests!", that may not push the boundaries of what you'd expect in an RPG, but has often pushed the boundaries of the technology available to it and of expectations for the series. It's a shame these remakes don't do any of that.
Still, they're good enough for what they are. I don't lament their apparently strong sales figures in the UK. I won't lament it if they have strong sales overall, when all is said and done. If there are a lot of folks who enjoy what they're getting and are having a good experience with it, so long as they're not being outright deceived or treated unethically, alright, then good for them. I'm glad they have that experience. Remakes of this variety aren't the experience I'm looking for, and, if they stay on this track, I'll evaluate them on a case-by-case basis, perhaps getting a future Black and White remake or perhaps not, but definitely passing up on an X & Y remake done in this same "faithful remake" style. That can join that Let's Go titles as future experiences that, hey, if other people enjoy, good for them, but which I'll pass by.
In the meanwhile, I'll continue to be optimistic for Pokémon Legends Arceus, which certainly looks like it's trying something different and experimenting more than the series has since... basically Sun and Moon. It looks promising. We'll see if it delivers.