Users appreciate the Simple CSS plugin for its ease of use, allowing them to add custom CSS without the complexities of native WordPress tools. The plugin improves website speed and provides the ability to apply CSS on a per-page basis, which helps maintain clean and efficient code. Users also like its theme independence, meaning custom CSS doesn't get lost during theme changes. However, there are drawbacks: the editor returns to the top after saving, causing inconvenience, and there's a lack of code hinting. Some users also missed features like a history function or line numbers visible in inspection tools.