The advances of micro-fabrication techniques allow for the manufacturing of micro-heat exchangers or micro-reactors. These micro-devices are characterized by a large surface-to-volume ratio and, hence, allow for the transfer of large heat fluxes or offer large catalytic surfaces for reactions. The design and optimization of such micro-devices heavily relies on correlations for pressure drop and heat transfer, as well as on information on the laminar/turbulent transition. As these questions are still discussed controversially in literature, a careful investigation appears highly desirable. This paper concentrates on rectangular stainless steel micro-channels with a hydraulic diameter of about 133 mu m. Three aspect ratios of 1:1, 1:2, 1:5 are studied, whereas the hydraulic diameter is kept constant. The average roughness depth of the channel walls is about 1-2 mu m in general, and specific channels are of roughness depth of about 25 mu m. Filtered and degassed de-ionized water is driven at pressure differences up to 20 bar through the channels, allowing for Reynolds numbers up to 4000. The measuring techniques allow for a highly accurate determination of the mass flow rate (precision weighting), the temperatures at inlet and outlet, the pressure drop, and the time-resolved velocity field (mu PIV). The measured quantities consistently show that the laminar/turbulent transition for smooth channels is in the Reynolds number range of 1900-2200, which is in agreement with findings for macroscopic channels. The influence of rough channel walls appears particularly strong for the micro channels of aspect ratio 1:5 (Reynolds number of about 1000). This raises the question of whether the relative roughness remains the relevant parameter at extreme aspect ratios. In this article, we focus on the mu PIV results.