I've been a huge fan of DDLC since the original but the additions here are just amazing (and you have also a good reason to replay it all).
In this review I'll only talk about the "plus" part, which is mainly side stories completely detatched from the main game, even though their existence is explained lore-wise, in their themes: you don't have the metagame or the horror part that was the main focus in DDLC, but instead a story about friendship that shows more multifaced and deep characters and puts them in a real-life setting. It doesn't have to follow the usual visual novel/anime tropes before deconstructing them because you already know the characters from the main game.
For this reason, I recommend playing it only after the original DDLC (maybe even after some decantation).
It's written so well that it seems almost didactic about how to handle complicated emotional situations, while not being too dramatic or exaggerated.
Its only flaws are the ripetitive structure of the stories (basically new member joins, the previous balance is ruined, they talk about it, problem resolved) and the fact that there's no real gameplay, just text and good music, even better than the original OST in my opinion.
Oh and you also have achievements and a new really cool and useful main menu

This game is ok, on the narrative side is really solid, pretty classic plot but executed very well: the endings were all consistent (not a common thing in RPG maker games) and the different take on the classic haunted house horror story is really cool.
The gameplay is meh, the puzzles aren't fun, mainly because you have a very generic objective and you're forced to wander at random to find things in order to progress.
It has some good jumpscares but other than that the atmosphere isn't really creepy (partially because there's no music in some parts of the game)