Celesphonia was the first RPGM adult game that I've played and I did overall enjoy it. The heroine is likable and her corruption process is rather well paced. Despite that I initially wanted to rate the game with only 3 stars. However, I feel that would be unfair to the amount of work put into it. A good chunk of reasoning behind this decision is highly subjective. What I consider a drawback might as well be a big upside to you.
Let's get the objectively good stuff out of the way first:
+ A lot of QoL features that make backtracking easy and exploring areas enjoyable
+ Good Artwork
+ A lot of content
+ Honestly, the story is quite fun, especially the small plot twist shortly in the final main story dungeon.
Now onto the things I had some issues with, even more so after comparing this game to its predecessor [caution, spoilers ahead]:
- The combat suit. It made sense in the beginning, but by the end I really wish I could fight in whatever clothing I desire (or, hell, even naked). Instead, it was always that magical girl outfit that I never really liked to begin with.
- Missed opportunities for NFSW content. There is something satisfying about returning to starting areas when your corruption has grown. I really wish Celesphonia could do something lewd in the cafe. Or at school without having to by hypnotized. I would rather have less defeat scenes in favor of more ways to make a total slut of Celesphonia in public.
- A bit of Moon logic with unlocking certain CGs. Riding a train while infected by a parasite. Those scenes were very well done, but taking a train is not really a thing after you deal with the invisible Monstrum - it literally does nothing, doesn't pass the time, does not take you from one location to the next, there is absolutely no reason to do it.
- The quest pacing is a little off. There are some missions that you get really early in the game, yet the materials required to craft items for those missions are not available until you are considerably further down the main story path. This made the first half of the playthrough a little frustrating as I had this constant feeling I'm missing something when in reality I wasn't missing anything at all.
- Also, speaking of materials. My god the amount of resources you can farm is overwhelming. I'm not sure I understand the need for this in a Hentai game with a battle system as simple as 2+2. This surely sucked some joy out of the playthrough when I had to farm stuff for a few items I thought looked cool
- Heroine doesn't seem to ever come to terms with giving in to the corruption. Even with 99 corruption points and LVL 3 Lust she still acts as if she's forced to do things against her will half the time. Now, this part is highly subjective. If you enjoy a rape fantasy, then probably that's right up your alley. It's funny how while I do not fetishize rape, this game's predecessor was very arousing with how much Flode enjoyed being used. But then again, Flode could always resist being molested and then she would say something teasing, along the lines of "oh, you want some of this sweet pussy? Too bad, I'm not in the mood right now". In case of Celesphonia I found myself feeling sorry for her in certain moments where I'd prefer to be horny.