One of my favorite series, and in my top 5 Yuri works (and I have read a very large amount of yuri). It gets a little melodramatic at times but there's a lot of artistry in this series that I think other comments miss.
This series subverts or avoids many of the Yuri tropes that drive me nuts. It is one of the few Yuri stories where it feels like two genuinely queer leads. Like a lot of Yuri stories, identity is one of the major themes in the work, but it informs the world and characters in an uncommonly cohesive way. Characters being queer matters here. It doesn't get too heavy, but it helps diminish the fetishization and weird tropes present in a lot of Yuri.
I found the two main leads very endearing, particularly Sumika. Her character arc is wonderful and maybe just me, but I found her some of the things she struggles with very relatable.a few melodramatic moments aside, I found her decisions and actions pretty reasonable given her insecurities and age. Kazama is more silly at times, but in the heightened reality of the series, it doesn't feel too out of place.
The series gets really depressing and angsty and the will they or won't they does get frustrating, but the highs after it are the sweeter for it. A decent chunk of the series happens after they get together which leads to both cute moments and cathartic character growth and resolution.
The best part of this series by far though is the gorgeous paneling. Do yourself a favor and buy the omnibus volumes from One Peace Books. Seeing the pages one by one on a reader really does not do the amazing layouts justice. This is a masterwork in using framing, panels, and transitions to supercharge emotions and get you to feel what characters are feeling. This one is a real treat to sit down and study passages of if you have any interest in comics as a storytelling medium or just read Understanding Comics once and want to see some breathtaking use of subject to subject transitions.
An absolute hilight of this is the flashback arc halfway through the second omnibus when Sumika realizes she's in love with Kazuma. The way the author uses paneling to evoke all the feelings of first love is masterful. From the way it depicts the sense of spacing out and disassociation when thinking deeply about someone, to the dynamic, euphoric joy of realizing your feelings, to the crash of anxiety as you realize what that entails.
I hardly re-read things, but Whispered Words/Sasameki Koto is something I reread at least twice a year. It's a sweet joy that grips me on every time with characters I enjoy revisiting, and art that amazes me with new details every time I read it.