Visiting Busa, Korea from Fukuoka

I have been to Japan 4-5 times and am planning to return again. Would it be worth my while to take the ferry or plane from Fukuoka to Busan, Korea? Just for the experience?

I can't answer your question, but we also have been to Japan about 5 times, and flew from Seoul to Tokyo once.

I looked into taking the fery from Busan to Fukuoka at the time, and the ferry looked reasonably nice.

So write again if you do it and let us know how it was!

I concluded it probably wasn't worth the time or cost, since we didn't really want to go to Fukuoka and it was far from everything else we wanted to see. But that might be different for you.

I looked into taking the ferry but I decided I wanted to visit Seoul. It was more time effective for me to fly Melbourne - Seoul - Japan (in Fukuoka out of Narita) - Melbourne, rather than ferry from Japan and train to Seoul. If you want to visit Busan then the ferry seems like an ok option, but I recommend visiting Seoul, I loved it.

It is only 3 hrs. by hydrofoil to Busan from Hakata. Buy the heavily discounted Korea Rail Pass package if you plan to travel in Korea by rail. Busan-Seoul is ~3 hrs. by the KTX bullet.
Also look into Nikkan Kyodo Kippu(Japan-Korea Joint Ticket) for Japan Korea travel. Be aware you may have a trouble finding a knowledgeable travel agent on this if further away from Hakata.