The paper presents the field performance of mobile multiphase flowmeters in more than 500 well tests during the last three years. The field tests were conducted on land or offshore in a wide range of field operating conditions, such as high flowing pressure, heavy oil, viscous fluids, gas and wet gas, gas volume fraction ranging from 10% to 99.6%, and water to liquid ratio frcm 0 to 100%. An in-depth analysis of the lessons learned shows the benefits and limitations of the technology in comparison to the traditional test separators and illustrates the expected range of accuracies that can be readily achieved in the field environment. Statistics from over 600 comparison tests provide a complete understanding of the accuracies of oil, gas, and water flow rates achieved with the multiphase flowmeter. The dual-energy spectral gamma ray venturi multiphase flowmeter requires the measurements of few basic well site fluid properties. A procedure, detailed in the paper, explains the utilization of a field kit specifically designed for this application when no prior information of fluids is available, which is frequently the case in well testing in new areas. The paper presents how the specifically engineered test-design software provides a rapid-decision tool to the selection of the best well testing solution (multiphase flowmeter or test separator). Finally, some operational recommendations for performance of well tests using multiphase flowmeters shares the experience acquired in these 500 operations performed worldwide.