I really don't know how to interpret these correctly. I know that they are supposed to be held a bit longer than a regular note, but whenever I try, it ends up sounding like a punctum mora. I don't mean horizontal episemas, those are easy to sing, it's the vertical ones that I do wrong. But it's odd though, because I think I can get it right when the episema is part of a salicus, but out of that, I tend to accent it too much and it sounds more like a held note than an accented one.
Not that it matters much. I've been told that, like the Graduale Simplex, the Vatican endition of the new Antiphonale Romanum no longer uses the vertical episema.I don't know id they're using any of the other possible accents for the notes, because I have'nt yet seen it.