Users appreciate the Relevanssi plugin for its extensive customization options, efficiency in improving WordPress search functionalities, and fast performance without impacting load times. The plugin is especially useful for large sites, e-commerce platforms, and those with PDFs and custom fields. Users commend the support provided by the developer, Mikko, who is responsive, helpful, and frequently updates the plugin. However, drawbacks include the complexity of the settings for novice users and high server resource consumption in some cases. Some users also noted limitations, such as difficulties with plural term searches and the need for experimentation to find the best setup. Overall, users highly recommend it for enhancing search capabilities.