Keikenzumi is an anime that starts with a premise that as a whole is entirely not original, but that has a small spice to it that makes it different than most of the others one could watch of this genre, which is the fact that the main girl is the experienced one and that already had plenty of relationships, while the main boy is the one that never did anything related to romance. Just because of that alone, there will surely be plenty of people that will hate this anime, because they are unable to see a main girl that is not a pure and innocent angel getting the spotlight, however, the ones considering watching this anime should not bother with that, for this is the literal one single thing that it is able to do properly: the interaction between someone with experience and someone without, and how each of them reacts to one another.
That is not to say though, that there are not plenty of others reasons to not watch this anime, because there surely are. From standard romances, it is probably among the worst ones of all, with not only very bad writing but also very bad investment on the anime, giving it ugly (though not the worst possible) animations, cheap style, lack of any significant music, and an horrible direction that cuts very relevant content from the light novel making some episodes utterly nonsensical. If that investment problem was all, it would not be a big issue, for romance animes tend to be less demanding in terms of animation and investment in general, to begin with, however it is not a case in which the quality of the animation sinks down the quality of the writing, it is a case in which both of these aspects hold hands and sink together to the bottom of the seas.
Every single character is extremely shallow and plain uninteresting, with motives that the ones watching would easily wonder if the author even gave it some thoughts before writing it. From common nonsense of characters falling in love after an exchange of 5 words on the stairs, to the famous group of losers that spend most of their day horny and makes that their entire personality, this anime has most of the standard badly written scenarios, and the development of the plot and its attempts to create dramas doesn not fall behind. Each drama that happens throughout the anime manages to be sillier than the one that happened before, to the point that one could wonder if they are really teenagers, and not middle-school children because of such silly mentality, with characters fighting over the most stupid and irrelevant things, and even risking their relationships over reasons so petty that it feels absurd, the type of thing that could be solved by literally talking for 20 seconds with each other. Perhaps that was an attempt to replicate the silly nature that teenagers of this age tend to possess, but it is made in such an absurd and exaggerated way that instead it feels like the show is about kids of 8 years old fighting between themselves because one took the toy of the other.
For those looking for something that simply has cute interactions, this anime may be something desirable, for it does have a few cute moments, but besides that; the fact that it follows well its premise of experience versus inexperience; and a few good original designs that manage to carry on even to the cheap animation, there is nothing else that this anime offers of valor. Every character besides the two main girls has absolutely zero depth, and while these two—Runa and Maria—does have a little bit more going for them, that does not save them from being badly written as well. Every other character—important characters, a relevant detail—is bad to the point that they do not even make sense, and if not straight from the beginning, they will disappoint and feel extremely childish or stupid at some point.