I picked these up on a whim figuring they would be easier to use than converters when playing with ink samples. Unfortunately, I was mistaken. They're easy to fill, but they don't seem to like any of my pens. The cartridges have a disturbing tendency to detach and leak inside the pen. So far, I've tried them on two new Ritmas, 2 older Nemosine Fissions, and a custom pen from a local craftsman and the longest I've had a cartridge say attached and not start leaking is 3 days. When they're attached, they work quite well. It's just that whole staying attached and not leaking issue makes these essentially useless to me. Time to brush up on cleaning out converters and allowing a bit more time between ink ink changes.