I like that it is easy to use with minimal configuration needed for basic reporting needs (e.g., number of visitors).
The output files are very large so we do not have much flexibility in how/where they can be stored locally. This was more an issue when using this tool along side our existing log analysis software rather than something specific about S3stat itself - but perhaps if you're looking at having two separate tools then there could also be some issues from trying to integrate both? It would really help us understand website traffic trends over time as well as giving greater insight into what users find most interesting or engaging. We primarily use these reports to look back after events such as campaigns where we want to determine which pages were visited during the campaign period compared against those outside of it; however, sometimes other metrics come up too depending upon your business model!