This is probably one of the better harems I've come across in quite some time. The thing that makes this manga enjoyable is that it knows what it is and doesn't try to pretend that it's anything more. The drama isn't overdone, the sex is pretty enjoyable to look at, and the decisions that the characters make fit within our understanding of how they've been presented throughout the story.
It's also conclusive, which is a big plus for me because a lot of the harems that I've stumbled upon tend to leave things open ended and not in a way where the buildup that is
felt throughout the story is justified. In Kanojo de Ippai's case, the build-up is very much justified - and you hardly feel it until the middle to end transition of the story. I think this is because things are kept rather light, even in moments where things are supposed to be "serious."
The characters are great, and as I mentioned earlier their actions make sense when considering what we've been shown while reading. When taking into account the length of this manga though, it's no surprise that some characters simply didn't get much time to develop. This is applicable both to some of the main female characters and some of the "side-hoes." There were also some characters in the story that weren't really looked at again, and were usually only brought back when they were convenient for moving the plot along. These characters kind of felt like a waste of time since they took away from the main female leads, but I digress.
Overall, I had a great time reading this. If you're looking for a harem with little to no drama and some smut in the mix, this is probably for you. If you want something more dramatic or less mature, I'd recommend looking elsewhere.