Had a play with this string. It was quite fragile for a 0.66mm gauge but power was okay and nice sound. Personally preferred the R4X Momone over this one, as it has more crisp sounds whilst maintaining durability (and better packaging).
Tried both of them, but on different rackets. Ryubi66 on Astec Pro800 (even balance, low3U weight) and Momone on RS Sickle Smash (4U slightly head heavy). Seems to prefer Ryubi, a bit better on repulsion (somewhat softer string than momone). A decent alternative at a much lower price than BG66Ultimax, about half price already strung.