Currently starting my third playthrough of Eternum, and I just have to say that it is easily THE best visual novel game I've ever played, surpassing Katawa Shoujo and Being a Dik, which are both amazing games in their own right.
The opening of Eternum sent me right into immersion (possibly because I had a high school friend who both looked and had a personality like Chang). The Kredon station scene set the mood so perfectly, it felt like I was right there, sitting on a bench on a sunny afternoon, waiting to see my old babysitter.
The cast of Eternum is an impressive improvement on the already strong character design of OIALT. Each main girl is uniquely interesting and possess layers of hidden depth for the player to discover.
The most amazing part of Eternum is how it seamlessly transitions into all sorts of different settings and themes, from cyberpunk to horror mystery to fantasy questing, and then back to normal life. It really feels like I'm playing a game within a game.
The only thing that sucks is the interminable wait for the next installment! :D