I had their #6 fine nib.
The nib performed surprisingly well, very smooth.
For some reason it didn't fit quite well in my Jinhao x450, probably their curvature a little bit different, but if you're bold enough and do not afraid to use some force (which on my jinhao's I don't) then, armed with a rubber grip, you can do it.
You might have noticed I use past tense here. Well, after about a year of usage one nice morning the tipping ball fell of the nib (half the tipping ball, actually), so I don't have it anymore.
I don't think it's common and, frankly, for it's cost I could replace those once a year. I still might.