Pokémon Brilliant Diamond & Shining Pearl aren't great games, but they are still enjoyable all-in-all. This is a given, considering what a wonderful region Sinnoh was. It's still a little disappointing given the fact that Omega Ruby & Alpha Sapphire before it fixed what was one of the worst games in the franchise and made Hoenn tolerable.
The Quality of Life is much better in these games than in the originals. One example is no longer being forced to lug around a Bibarel as you had to in the originals; HMs are now a part of the Pokétch and are unlocked as soon as you obtain their corresponding TMs.
The graphics are of debatable quality, but the overworld chibis are quite cute and the in-battle trainer models are well-crafted and faithful to their original sprites. In particular, we get to see the Battle Girls and the female Cyclists in all their 3D glory. Oddly though, they screwed up with Mars' model, and she's no longer as attractive.
The Underground also got a major rehaul, and instead of simply mining and decorating bases, there are now special areas where you can encounter wild Pokémon, including those that aren't in the DP Sinnoh dex (Magnemite and Houndoom, both of which had to wait until Platinum to enter the Sinnoh dex, come to mind).
While the Union Room is back, they made it so that you are able to access it anytime (after your initial visit) by pressing the Y button.
However, many of the issues from the original Diamond & Pearl that Platinum addressed have come back in these games, namely the underleveled NPCs, slightly weaker plotline, and the lack of Fire-types in the region (discounting the Grand Underground). Similarly to the previous two entries for the Nintendo Switch, the Exp. Share is mandatory. Just like the (sadly underrated) Sword & Shield, affection is also mandatory, despite Sylveon not being in the game.
Overall, the game warrants a 7/10. Good but not great.