In recent years, power consumption has become one of the most important design criteria for microprocessors. CPUs are therefore no longer developed with a narrow focus on raw compute performance. This means that well-established processor features that have proven to increase compute performance now need to be re-evaluated with a new focus on energy efficiency. This paper presents an energy efficiency evaluation of the symmetric multithreading (SMT) feature on state-of-the-art x86_64 processors. We use a mature power measurement methodology to analyze highly sophisticated low-level microbenchmarks as well as a diverse set of application benchmarks. Our results show that--depending on the workload--SMT can be at the same time advantageous in terms of performance and disadvantageous in terms of energy efficiency. Moreover, we demonstrate how the SMT efficiency has advanced between two processor generations.