Far enough to speak some on somnium now. Main thing to get out of the way is that this isn't really like 999 or Danganronpa. The gameplay elements are nowhere near as frequent or involved as the later and it doesn't have puzzles in the same way as the former. Think of it more as a straight Visual Novel with Adventure Game elements, and even those are more like a bizarro version of the Telltale scenes where you're walking around interacting with stuff.
However, I'm still really enjoying it. It's more of a standard procedure murder mystery, but it's never on you to solve things and the game all focuses around one case. Also your left eye houses a sentient AI named Aiba and you can enter the dreamworld of various characters at times. Plus some sideline discussions into Egyptian mythology, secret societies, alien intelligences, parallel worlds and all that stuff. It's no Dear Esther by any means.
There way you choose to solve puzzles in the dreamworld is where your story branches come from. The way your actions in the Somniums can affect the real life story works as a system and you can jump to any scene at any time through a handy flow chart, but the suggestion is to play through to one ending before messing around too much.
The bulk of the game is traveling to a scene in the 'real world' and interviewing people, with locations having optional hotspots to look at that might give extra dumb jokes or dialogue. There are looots of dumb sex jokes and puns, btw, talk of lanterns feeling like breasts, stereo systems looking like dicks etc, some funnier than others. When you've asked about everything you can, you're allowed to move in to the next destination and repeat. Sometimes the order is fixed, others you have like 3 or 4 to choose from, but so then you just hit them all and then progress again.
I can see people who go in expecting court cases or tense murder trials or brain testing logic puzzles being disappointed, because this isn't that, but it is another complexly woven and well told story from Uchikoshi, and I'm definitely enjoying being along for the ride.