I’d say the side stories are worth it if you like extra bits about the characters.
カサブランカ·メモリー
Nice story featuring Rei. Takes place during the Dark Kingdom Arc.
かぐや姫の恋人
Luna’s story!
This is the source material for the Sailormoon S movie, which was my first exposure to Sailormoon in Japanese. (I rented it on VHS from a little Japanese shop near a friend’s house way back when, and I may have accidentally utilized two VCR’s and a blank tape… Don’t worry, I’ve since bought it on DVD/BR!)
I don’t think I can really use having watched the movie in the 1990’s as a barometer for my liking it today, but I’m absolutely looking forward to reading the source material.
受験戦争編 第1詰 まこちゃんのユーウツ
It was nice to get a chapter centered around Makoto, but I felt it was a bit weak.
受験戦争編 第2詰 亜美ちゃんの初恋
I was looking forward to this one since I felt the anime adaptation was pretty bad. As it turns out, the source material wasn’t all that great, either. (Which is a shame, 'cause Ami’s my favorite.)
受験戦争編 第3詰 レイと美奈子の女子高バトル
This one I really enjoyed. I’ve taken something of a liking to Minako as I’ve read through the series, so it was nice seeing more of her. And even better that there was a lot of interaction between Minako and Rei, something that we don’t really see otherwise.
My main complaint with the Makoto, Ami, and Rei/Minako stories is that they have baddies shoehorned into them.
ちびうさ絵日記 - 転校生にご用心!
I enjoyed this one. Of course, I extra enjoyed it because it made use of the Japanese word for “vampire”, which I had just learned via WaniKani (and have since forgotten… But I do know it when I see it, as I recognized it when it came up in another manga months later, right before it was up for SRS review.)
ちびうさ絵日記 - 七夕にご用心!
I enjoyed this one as well, although some of the fairy tale parts were hard for me to follow with my limited (for this story) vocabulary.
ちびうさ絵日記 - 虫歯にご用心!
I found this one to be a bit so-so. I feel it introduced a side of Usagi’s Papa that I’d rather have not known about…
ちびうさ絵日記 - ヒミツのハンマープライス堂
I haven’t read this one yet.
パラレルセーラームーン
I haven’t read this one yet. This one is included in tankoubon release.
As for Sailor V, I’ve read the English release of the shinsoubon, but for some reason I don’t remember anything from it beyond the first two pages from past failed attempts to read it in Japanese. (Maybe I read through it in English too fast?) So it’ll be like all new material for me to read through in Japanese.