For a game that shouldn't be my cup of tea (not a fan of NTR), gotta say that I've enjoyed it.
For starters, the pure love route isn't, as people already said, an afterthought. Almost half of the scenes are in that route, it has different final bosses and the story changes quite a lot.
Second, I do like how they treated the dark route. Unless you let one character break the heroine completely (and you have plenty of space to see all the scenes and keep her unbroken), she still has a shot at having a happy ending. Not only that, but three of the main characters, included Flare, develop a LOT in the dark route (Flare is kinda static in the pure love route, because there she just keep winning, so she isn't forced to mature and compromise).
The gameplay per se has an interesting idea... that is rendered moot once you learn Stun Thrust. That move can win EVERY battle in the game, without the need to heal yourself, so long as you kept leveling (and seeing how it can oneshot every normal enemy, and in the last two zones every normal enemy give you enough exp to level up, you'll easily end at level 50+, more than enough to curbstomp everything in your way). A pity, but if you can resist to spam Stun Thrust, the idea behind the Rock Paper Scizors system makes you think a bit more in the battles (if used properly still lets you curbstomp enemies, but in a more strategic way, instead of just letting the Z button pushed and see everything die)
Gameplay aside, my only complain, and it has more to do with my tastes, is how little anal and multiple penetration scenes are. Technically you can develop every part of Flare to Level S, but, at least in my case, I couldn't do it without letting her broke, because there are barely any scene that trains her ass, and once you beat the first boss, the only one you can spam is one of the scenes that break her little by little, so I was forced to finish my Happy dark route with an S rating on everything but her ass, which just got an A (and I needed just 9 more times with her ass used, the scene I'm talking about adds 5 uses)
Usually I'd give it a 3, or i'd specify that it's really a 3.5 instead of a 4, but the fact that even with being a NTR game I like it pushed it's score that half point for me.