i'm planning a trip to Czech/Germany/Denmark/Netherlands in October and am looking at train options.
I agree that first class is a worthwhile option in or to/from Germany on
ICE (IC trains are another matter so check the DB timetable carefully). There are many savings fares which make it advantageous to a leisure traveller.
However travel in Denmark and NL on domestic trains is another matter. The carriages are a bit older and it's sometimes difficult to tell what you are paying for.
Fully endorse travelling 2nd and taking a meal or a beer in the German/CZ dining car which is nicely traditional with table cloths and excellent service