Got these to use in my rOtring 600 series pens. These write great! Has become my regular refill now!I've only been using one of the five refills so far, but it writes well and I haven't experienced any problems with inconsistent flow yet. We'll see if I remember to update my review when I've used it for a while, but I'm happy with it so far.This were delivered on time. The line is nice and fine and ink does not smear (I am left handed, so this is an issue). I cannot comment on longevity because I have not had them very longI have had a number of Parker ballpoint pens for years but recently I have moved on to using gel pens. I had purchased a Cross retractable gel pen but they do not offer a fine tip refill. I was delighted to find these refill replacements with a fine tip for the Parker pens. So far they have proved to be very satisfactory and I am very happy with the performance.Correct product at a good price.Just pulled the first refill from the pack and the refill does not write!! So frustrating!! What a waste of money!Monteverde gel refills used to be the best, but quality control has gone down the drain!I have had several refills that leaked all over the place, gumming up the inside of whatever pen they were in plus leaving blobs of ink everywhere.I’m done with this brand. Schmidt is wat better. Too bad !Very messy, always leaking.