Interesting. I purchased FLVI when it was launched because it will probably be useful for certain things that are maybe not possible or tricky to do in Glyphs. But I don't plan to go back to it as my main font editor.
My impression is that it is very fast. (Maybe it depends on hardware.) Document sizes for the same font data appear to be smaller than with Glyphs, but they are not human readable. It has a lot of interesting new features. On the other hand, the UI and workflow of the new FontLab is so different, both from Glyphs and previous versions of FontLab.
I've changed primary font editors twice in the last five years (FontLab -> Robofont -> Glyphs). It's hard to imagine it would be worth the time and effort to do it yet again, especially given how happy and efficient I am working in Glyphs.