We lived in Germany from 1987-1991 and traveled by train a lot. We never went first class. We did take night trains a few times and the difference between second class couchettes (six persons to a compartment and no way to lock the door for security reasons and first class sleeper is for two persons with private toilet and sink.
We did the first class sleeper when we traveled back from Denmark after our wedding, it was great. Still, we did the family thing with couchette sleepers (usually slept with wallet and passport under my pillow).
Also, I remember traveling on one train in Italy that had standing room only (second class). We wished we had done first then.
Still, most of the time second it fine.