Herringbone microstructures are a very promising class of flow promoters to passively enhance heat transfer in microchannels by efficiently triggering helicoidal fluid motion. A host of applications are envisioned to benefit from heat transfer enhancement in microchannels, including microfluidic interlayer cooling of 3D electronic chip stacks, or advanced concepts of integrated cooling and electrochemical power delivery. Here we investigate the cooling performance of microchannels with such flow promoters and show that the Nusselt number reaches an average value of Nu = 36.6 at a Reynolds number of Re = 510 for our best performing design. This result constitutes a fourfold improvement in heat transfer capability compared to a plain microchannel. The fluid temperature is assessed optically using micron-resolution laser induced fluorescence (μLIF), while the wall temperature is measured with on-chip resistance thermometers. In addition, we determine the pressure drop originating from the presence of the herringbone flow promoters. By taking into account both the beneficial heat transfer enhancement and the adverse increase of pressure drop in a non-dimensional figure of merit (FoM), we demonstrate a significant performance enhancement of 220% at Re = 350 using herringbone structures for heat transfer augmentation compared to a plain, unstructured reference microchannel.