This paper presents five domain-specific Retrieval-Augmented Generation (RAG) applications developed based on real-world use cases across five domains: governance, cybersecurity, agriculture, industrial research, and medical diagnostics. Each system integrates multilingual OCR, semantic retrieval via vector embeddings, and domain-adapted LLMs, and is deployed via a local server or cloud API to meet user requirements. A web-based evaluation with 100 participants evaluated the systems across six dimensions: usability, relevance, transparency, responsiveness, accuracy, and recommendability. Based on user feedback and development experience, we documented 12 key lessons learned that highlight the technical, operational, and ethical challenges impacting the practical application of RAG systems. This paper aims to address the lack of empirical research on the development and evaluation of RAG systems based on real-world use cases.