The paper aims at assessing the effects of combined voltage and/or current synchrophasor measurements, and their associated uncertainties, on the accuracy of state estimators adopted in distribution systems. Such an assessment is first carried out with respect to a generic transmission line with the purpose of determining the combination of voltage and/or current synchrophasor measurements that provides the best accuracy of the estimated quantities. A comprehensive analysis on the impact of different measurement uncertainties and operating conditions is included for this specific case. In order to derive general conclusions, the study is then extended to a distribution system composed of the IEEE 13-bus test feeder. For this case, we perform an a-posteriori assessment of the probability distributions of the estimation errors by using a discrete Kalman filter state estimator fed with noisy voltage and/or injected current synchrophasor measurements.