I've gone through three hundred needle tips across my two units and four others already. I never count how many shots each one has delivered. Those remarks on precision dropping after repeated use caught my attention. For the models with the metal screw plunger, where a spring controls the force and you turn the knob to set the amount, it either functions properly or stops altogether. So suggestions that precision fades over time seem improbable, basically nonsense for these particular devices. I avoid using pens altogether since there's no point. Still, without solid proof of a precision problem, I dismiss that notion. I figure one day the mechanisms will break, maybe the spring or the release mechanism, yet they could easily last longer than I do. I own three more versions that seem well made. I got them thinking I might switch to thicker liquids and wanted sturdy hand-operated injectors. I haven't tried that so far and actually favor the automatic kind for clear solutions. The lower-cost versions probably develop play in the parts after normal use because their plungers are plastic and you can push them manually.