Immersive virtual reality serious games (IVRSG) have revolutionized education and training by providing impactful user experiences (UX). IVRSG enables users to interact with environments that enhance learning and skill acquisition. However, the IVRSG environment presents challenges when evaluating UX. This study introduces Virtual Reality Evaluation for Experts (VREX) guidelines for UX evaluation in IVRSG, addressing issues such as immersion and comfort. The VREX recommends a systematic evaluation encompassing pre-evaluation familiarization with equipment, heuristic assessment, task analysis, and feedback. Key metrics such as immersion are emphasized to ensure user engagement. The study highlights the need to address physical challenges like motion sickness considerations to enhance user comfort. A case study demonstrates the implementation of VREX in evaluating two flight simulation games, revealing how experts identify design flaws. It concludes by advocating for standardized evaluation systems to address the evolving VR technologies. This work offers a resource that aims to optimize UX.