Very easy to fill and I love thst I have three pens with me at all times: One by my bed, one in my purse, one at work. I’ve been using the oil for about three weeks now, and my horrid cuticle problem — lots of hangnails and tough sides — have disappeared. I have extremely soft nails that peel all the time, and professional manicures make my problem worse. For the first time ever I have been able to do my own manicures and I am thrilled with the results. Love the oil and these pens.