The 'first' installment of Higurashi Rei is rather slow and uninteresting as a part of the wider Higurashi universe; however, it's very necessary as a setup for the next two arcs of Rei.
Now, I'm just going to talk briefly about how well it does as an add-on to the original. When it comes to this, it does well in some areas and terribly in others. It builds on the existing story strictly for Hinamizawa and Hinamizawa alone; this is obviously great, but the original cast barely appears, and the way we get most of our information about them is through dialogue by the new main cast - their children. It's not even established who certain characters from the original conceived these children with, and with the structure of most Higurashi arcs being 50% Slice Of Life, it was a missed opportunity to not give us at least a little bit of fanservice around the original cast. It was just a bit disappointing.
It felt like I was reading Onikakushi again but with a new coat of paint, which both works and doesn't considering its status as a story that takes place 40 years after the events of the original. It works because I could tell that, while it was a rather underwhelming in terms of characters, with there only being three that I liked, its purpose was to set up the next arc while introducing us to the characters and plot. It's not bad, just a bit underwhelming.
At least it's not Gou/Sotsu.