Next-generation rotary endodontic instruments with improved clinical performance are being fabricated from special pseudoelastic NiTi wire (termed M-Wire) that has a nanoscale martensitic structure. In this article we report etched microstructures, Vickers hardness measurements, and SEM observations of as-manufactured and clinically-used GT® Series X™ rotary instruments, with comparisons to rotary instruments fabricated from conventional dental pseudoelastic NiTi alloy. The next-generation NiTi instruments have significantly higher Vickers hardness and much less evidence of clinical wear compared to conventional NiTi instruments. Etched microstructures show a martensitic structure, which is consistent with our previously reported STEM observations. Differential scanning calorimetric analyses have shown that the manufactured instruments have similar phase transformation behavior to starting M-Wire blanks. The properties and clinical performance of the next-generation NiTi rotary instruments arise from their special martensitic structure. More TEM studies are needed to elucidate details of this complex microstructure and effects of clinical use.