OK, we just got back from the trip and did 3 overnight trains. I was a bit concerned since about 2/3 of the people here said they had bad experiences. But . . . my friend and I loved them and slept like babies! Now, we had our own private double sleeper compartments (that locked), not couchettes.
The one thing that surprised me was how very, very small they were! The beds were good sized and comfortable, but the "aisle" where we walked was so small that only one person could stand there at a time. The Paris-Rome train was not too clean (especially the toilet room), but the Venice-Vienna and Vienna-Frankfurt were both quite nice. We got nice breakfasts on all of them and once we got served 2 small bottles of wine before bed. In the last train we had our own bathroom and the shower worked great even if the bathroom was pretty tiny. So, I would definitely travel on them again. My only complaint would be that all the train compartments were so hot. There was no way to turn the heat off. We had to keep the window open (which made it hard to shut the blind) because we needed a breeze to cool down the room. Otherwise it was a great experience and a nice way to travel.
The day trains were also quite nice. On the one from Frankfurt-Paris we were served a lovely lunch with wine. Can't beat the comfort and great scenery either.