This manga is downright amazing. I didn't think much of it at first, simply viewing it as a typical romance manga with a tsundere female love interest who constantly torments the MC, similar to Chitoge from Nisekoi.
This was true for the first half of the manga, but Yuiko's awkwardness and jealousy charmed me which fueled me not to drop it. Furthermore, Tomoya and her had many cute romantic moments and arguments which were enjoyable to read.
However, it was in the later part of the manga where I realised just how much of a masterpiece this was. I'm trying not to spoil it so
I won't be able to cite any examples, but basically, both Tomoya and Yuiko begin to mature and start thinking about their future together, and how they can shape it by making crucial but sacrificial decisions in the present, even if they may involve being able to see each other less.
Furthermore, as Yuiko was 'seperated' from Tomoya during the later part of the manga, we are able to see how she develops as a character and how she learns to stop relying on Tomoya, step out of her comfort zone, and try to form her own circle of friends. Needless to say, she was successful, and it was heartwarming to see how she enjoyed life with her new friends.
Although it was heart-wrenching to have to watch Tomoya spiral into loneliness and slight jealousy, the new matured Yuiko manages to comfort him, express how grateful she is to him and tell him her true feelings regarding their relationship.
You can either view this manga as a typical romance manga (which it does well), or as more of a life story, with the real protagonist being Yuiko-san. It reached out to me as a charming story of a awkward high school girl's development into a passionate and mature woman of today, with plenty of help from her extremely caring boyfriend and their circle of friends, hence explaining the name Today's Yuiko-san.
In conclusion, this manga really shines in comparison to other romance manga, as Kyou no Yuiko-san is able to provide the reader with 2 different 'routes' or interpretations of the deeper message it is trying to convey. Once you are able to realise the existence of both paths, it really proves to be a masterpiece.