after ordering my v4l premium cartomizers I decided to put them through my own special torture test the test started with five ML's premium menthol propylene glycol juice from v4l. The next step was five ML's of my own blend of menthol vegetable glycerin juice after that I did a complete boil (5 min boil, 2min dry,3min boil,2 hour dry) after that I refilled with more menthol juice vg....after all that (which took about a week) those puppies keep on working no burnt taste no lack of vapor just great vaping hope to make it about a month with this cycle gotta thank v4l for their great products!!!