I took in an afternoon viewing of BR2049 yesterday (opening day). Background: I rarely go full fanboy on media stuff but I've long been a fan of the original movie, and have it on almost every media possible (RCA Selectavision disk, heh), so I was very geeked up to FINALLY see a sequel. To warm up I reviewed every version in the 4-disc DVD set, and I'm glad I did; there are details in the various versions which are important in the new movie. Run time of the movie is about 2:35, then 8-9 minutes of credits. No aftercredit scene. The movie is very deliberately paced (many reviews use just that phrase to describe it), and action sequences are spaced thru the film. Music is not much of a throwback to Vangelis, but there are still many familiar sounds; as SpitfireJoe notes, the audio is thunderous. I viewed it in 3D; is it necessary? My take is, yes, to properly experience the virtual character that is Ryan Gosling's sidekick you need the 3D; other holography is used at points throughout and the 3D helps to 'feel' it.
Bottom line, if you're a fan, definitely go. Many loose ends are tied up, and a few are created. It's a very satisfying sequel.
If you're neutral on the 1st movie, there are still a ton of visuals and sci-fi concepts to absorb and ponder. Reviews have noted the outstanding cinematography and there will be a few Oscar noms for this movie, even if they might only be technical ones.
Speaking to the room, if you've never seen the first movie, it would help greatly to do so beforehand.
I will echo the news that my viewing was very lightly attended but then it was a 3:25pm showtime.