First off, I have a fear of heights, but the High Roller was A-ok! Went during the day time when prices were cheaper. I enjoyed it a lot. Was able to see as far as the mountains. Takes about 30 minutes to get a full rotation. You barely notice that it's moving, very gradual. Make sure to use the bathroom and don't drink too much before you go on. Night time I imagine would be just as nice, only you wouldn't be able to see far off, but the Vegas skyline would make up for that.
The units can hold up to 40 people, but I seriously don't see it having to be that cramped. Unless your group is just that big. They are spacious and air conditioned. It's worth doing at least once. Not for the thrill, but for the view.