This to me is another one of those really well done sci-fi mystery shows that TNG did better than any of the other series.
I thought the music was atypically moody for the show and the cinemtaography gave an almost dreamy surreal effect to the entire episode--quite appropriate I thought given the subject matter. To this day one of my favorite shots is of the Enterprise coming into view in the first seconds of the show with the light from the binary stars sweeping into the camera. In fact the entire episode was quite successful creating the feel of space being actually eerie and the Enterprise was out there all by itself facing some unknown mysterious force that was crippling their engines and the crew.
The episode was outstanding when it came to the creep factor--the young ensign hearing noises on board a ship where the entire crew killed themselves, the corpses darting up surrounding Beverly in the morgue scene, the haunting voice calling to Troi in her dreams.
I certainly didn't know what was going on until it was revealed and I thought the riddle of hydrogen was clever as was the idea of REM deprivation being the cause of the crew's odd behavior.
Patrick Stewart and Brent Spiner gave great performances. I especially liked the scene with Data where he recalled the fear he had for losing control of his mental faculties. It played nicely off what he went through earlier in the season when he was assimilated and while obviously not intended at the time works nicely into what we saw in AGT where he was suffering Irumodic Syndrome.
Lots of nice little touches as well--seeing another ship and bridge, Guinan pulling out her gigantic rifle from behind the bar, the touch of continuity from The Best of Both Worlds when Data offered up the deflector discharge to dislodge the ship.
Despite knowing how the episode plays out, I enjoy watching this one over and over. I know the crew and writers didn't care for it but it is one of my favorite season four episodes.