A look at the current timetables shows that the trip from Hamburg to Copenhagen via the Fehmarn route is just on 5 hours, with a 65 minute section from Puttgarden to Roedby of which about 45 minutes is the train on ferry trip. The other route via Frederica (with a change of trains there) takes 5:15. Transferring from Train to Ferry to Train would probably add an hour if the trains didn't take the ferry.
The huge abandoned rail yards at each end point to much more than a few passenger trains in the past (before the Great Belt Bridge was opened in 1998).
I suspect that when the day arrives that the Ferries need to be replaced, then that will probably end the experience - if the proposed Tunnel Construction doesn't end it first. A single train journey via the Great Belt Bridge/Tunnel would probably be the replacement, and would allow for trains longer than the current 4 cars imposed by the ferry length.
Both routes are worth the trip, which is what I did, coming back via the Great Belt - you also get to experience the 'long spiral loop' journey through Flensburg and the trip over the Rendsburg Bridge (it crosses the Kiel canal and is also approached via a 'spiral loop' on the north side).
And the 'Archeology' on the German side of both routes is amazing.
I have found some photos and a poor quality video of my trip on 16 October 2008. They guy in the red jacket in one photo is me. Interestingly, the single DB driver was replaced by about 6 DSB crew at Puttgarden, who all crammed into the cab and thus forward video wasn't possible.https://flic.kr/s/aHsmxoAm88