Accurate and efficient power flow (PF) analysis is crucial in modern electrical networks' efficient operation and planning. Therefore, there is a need for scalable algorithms capable of handling large-scale power networks that can provide accurate and fast solutions. Graph Neural Networks (GNNs) have emerged as a promising approach for enhancing the speed of PF approximations by leveraging their ability to capture distinctive features from the underlying power network graph. In this study, we introduce PowerFlowNet, a novel GNN architecture for PF approximation that showcases similar performance with the traditional Newton-Raphson method but achieves it 4 times faster in the simple IEEE 14-bus system and 145 times faster in the realistic case of the French high voltage network (6470rte). Meanwhile, it significantly outperforms other traditional approximation methods, such as the DC relaxation method, in terms of performance and execution time; therefore, making PowerFlowNet a highly promising solution for real-world PF analysis. Furthermore, we verify the efficacy of our approach by conducting an in-depth experimental evaluation, thoroughly examining the performance, scalability, interpretability, and architectural dependability of PowerFlowNet. The evaluation provides insights into the behavior and potential applications of GNNs in power system analysis.