After trying it for a few hours (2 sessions) here are my impressions:
It’s indeed rougher than BG66UM – but not by much. That’s the only downside (for me) I found; it’s not as rough as BG80 for instance, which makes slicing the shots less impressive.
Other than that, it’s awesome. Excellent repulsion, nice hard feeling (harder than BG66UM). Can’t comment on tension retention or durability that much, but so far it seems okay; better than BG66UM. The sound it makes when you hit a big smash is just unreal.
One of my new favorite strings; I even like it more than BG80. Have to test it against R4X110, if it wins, we have a new No. 1 (pun intended). 