I think the SuperS specials are a great addition to the series itself. For Haruka and Michiru fans especially, you get some of their best scenes together in their special especially with them not being in the SuperS season itself. The episodes are fun and fast paced. They also show off the peak of the dreamy disco artistry that Sailor Moon achieved in later seasons. I definitely recommend giving them a watch. There is nothing too deep that happens, just basic enemy appears/fight plot. They are not to be taken too seriously, just fun filler to grow closer to the characters and get deeper insight.