I did the return journey a few months back. If you ask me, it's far more pleasant than flying. To begin with, I can board the train in Hua-Hin and return directly to Hua-Hin, which means I don't need to endure a trip to Bangkok. I can't recall if you can buy a return ticket in Hua-Hin or not. I had a one way ticket and bought a ticket for the return trip in Panang, direct from the ticket office located at the ferry terminal.
The immigration routine is a breeze and really no hassle at all unless like me, you come from any country in Africa in which case you'll be sent to a separate queue on the Thai side, where you'll be asked 101 questions and probably have to show proof of finances. After all, as far as most Thais are concerned, Africa is a country rather than a continent full of countries. Still, immigration at the station is better than anywhere else - in and out in a matter of minutes.
Note: I'm not sure how many trains there are each day, but if you take the one which is supposed to leave Hua-Hin at around 20H00, it will usually be at least two hours late but of course you can sit across the road and have a few beers while you wait. Don't automatically assume there'll be a restaurant carriage either. Sometimes there is, sometimes there isn't.
As others have pointed out, ignore the taxi bandits at Butterworth. The ferry terminal is only a five minute walk from the station, and the crossing is a lot cheaper and quicker compared to a taxi.
All round, a pleasant relaxing experience, enjoy.
Don't try to impress me with your manner of dress cos a monkey himself is a monkey no less - cold fact
