I recently had an ersatz Halla Bong. The shape and overall texture/taste were quite similar, though this one was called a 'Sumo'. Halla Bong means 'The nipple at the top of Mount Halla'. Anyhow, the skin was way more porous and lumpy (not shiny and tough like a HB), and the after taste was a lot sweeter than a HB. But it was good.
I will eat a Halla Bong on Jeju again. Onje? Cheel wol. Amado.
Here are my musings on Halla Bongs from exactly two years ago. Almost. http://harfangperdu.blogspot.ca/2011/03/loves-me-them-halla-bongs.html