If you have a hard time sleeping in unfamiliar places, I don't recommend it--people going in and out will drive you crazy...both because they're getting on and off the train, or because you're bunking with a smoker who needs a cigarette periodically. IME, however, getting a good night's rest in a 2-berth sleeper was quite easy.

I've booked the 6-bed couchettes before, and ended up in the very top all but one time. If you're at all afraid of heights, I don't recommend it. They are quite narrow, and I'm not a large person (and was even less so then), but I felt like I was going to fall out all the time. On the other hand, yes, the top is more private.

We ran into some people who were chatty, one who was odd but harmless, and everyone was pleasant enough.