When evaluating business card magnets, the most critical factor is the thickness of the magnetic sheet, typically measured in mils (1/1000th of an inch). For standard business cards, 20 mil to 30 mil is the industry standard; anything thinner may feel flimsy and have poor magnetic pull strength. Additionally, ensure the surface coating (usually UV or aqueous) is high-quality to prevent ink fading and scratches, and verify that the adhesive backing (if purchasing DIY magnets) is a high-tack acrylic to prevent peeling.