I have one. It's has been great so far. I will tell you that if you need to adjust it for a different length, pull the mandrel out first and don't try to push in. With mine anyways it jammed pushing it in and made it a task to get it back out without damaging it. This is the one I have here. It was also the one recommended to me here.
http://www.woodturnerscatalog.com/store/Pen_Making___Pen_Making_Tools___Adjustable_Pen_Mandrel___adjust_mandrel?Args