I haven't gone through a complete list of the refills to try them out, but the ones I have fit well. I've used the Pilot G2, Uniball Signo 207, and the Montblanc rollerball. Removing the tip section is pretty easy and smooth and the added o-ring provides some tension to keep it from unscrewing with consistent use. The last thread when screwing it together takes a bit of extra pressure to twist.
Using really inky refills like the Montblanc rollerball gave me trouble as it would in any retractable pen as it isn't really intended for it. I ended up with a big blue stain in my pocket as that refill, although it fits, is not really meant to be stored upside down with the tip open and exposed. Learn from my mistakes! Or just proceed with caution when you choose a refill like that. Any normal retractable refill should be ok in theory as the companies develop the inks and refills to function in that way already.
I enjoy the design element of the grip section. There is something weird about me that I really like metal pens that have some very defined grooves machined into the barrel for either aesthetic or function. Their choice of the three lines in the barrel at the break point was well done.