There's some reviews up, but from what I've heard it's not great.
This. Out of the 5 available review (which you can view here: link), 3 of those reviews give the product a 2 out of 5.
Just to give you a bit more information on Koa, it was designed to be more of a versatile couples vibrator that can be worn as suggested and also on the hand of either the guys or the girls. Due to this, it is quite hard to compare the 2 directly.
We suggest at PicoBong that if you want just a bullet ring then perhaps purchase something else but if you would like a versatile little couples vibe that can be used in different ways including using it as a ring we suggest that Koa will fulfill all your needs.
Thanks very much and we hope you enjoy whatever you purchase