First, let me say that traveling by rail in Europe is a great way to get around. We've spent probably 5 weeks traveling by train to various parts of Europe over the last few years. They are always on time, very clean, comfortable, and quiet.
We did one overnight in a sleeper berth from Munich to Florence. It was fine, but it's not always the case. Our son and his wife took our experience and decided to try the same thing from Munich to Rome - bad mistake. They took pictures of their berth and it was rather disgusting. After exchanging letters with Rail Europe, we got them a full refund because of the problems. Our berth was comfortable and adequate. We spent the extra money so we had a bathroom, which was a good choice. There were no dining facilities onboard, so we had the forethought to bring food and beverages with us. The beds were comfortable and clean. The good thing is, it did save us a night's lodging and we did not waste a day traveling. But there's nothing to do, so your limited to staying in your small berth. And while I slept okay, my wife did not because the train makes several stops during the night, so the stops and starts always woke her up, along with the noise of the train. Because of our overall experience, we chose not to do the overnight berth again, choosing instead to travel during the day where we enjoyed the scenery, being able to get up and move around, the food, and the people. Plus, the fast trains during the day are more comfortable than the overnight trains.
Another suggestion; make sure you purchase your train tickets before you go. We sell Rail Europe passes and always recommend them as the only way to go. Costs are alot less, they're always first class tickets, plus you have Rail Europe to assist you should any problems arise. Their website is easy to use and when they send your tickets, they also provide a wonderful book with all the schedules.
We once decided on purchasing a train ticket while we were in Europe for a trip we hadn't planned. The cost was considerably higher than if we had bought it ahead of time through Rail Europe.
While train travel in Europe is overall much nicer than in the U.S., the sleeper berths on the U.S. trains are much nicer.
But we've been on the Munich to Amsterdam train as well as the Venice to Munich train. The one between Munich and Amsterdam is a high speed train that is very nice. The one between Venice and Munich is also nice, but because it goes through the Alps, it's not as nice as the high speed train and definitely not as fast.
When we were going from Venice to Amsterdam, we decided instead of taking the night train, we would just spend the night in Munich and finish the trip the next day. This turned out to be a good choice as we found a really nice hotel about 3 blocks from the Munich train station and then found an awesome brau haus (beer house) where we enjoyed some great beer, very good food, wonderful company, and great local music. We both agreed it was much better doing it that way than to take the overnight train.