I know a few friends who have seen the VN and they've recommended it to me. I don't think you should judge something just because its an eroge. Fate/Stay Night and Saya no Uta are a few popular eroge with really great stories.
I mean that Grisaia just isn't my cup of tea. Maybe it have great story or anything, I'm just not interested from that plot summary. I quick to judge thing like that.
As for eroge, yes, there are a lot of them with great story. But for most of the time, great compelling story is just a nice plus, not the main attraction. I play Sono Hanabira ni Kuchiduke wo and i love it, but I also think that if they toned down the sex and up the drama, it'd be better.
If that make me sound prudish, I don't hate porn it just goes to different stomach.