All connected file system clients see strong, read-after-write consistency. Most file operations are synchronized to S3 within a few minutes of completion.
Write conflicts between the file system and S3 should be rare (by definition, applications shouldn't yet be designed to do this because Regatta doesn't exist). We do some tracking of the object etag to at least throw an alert if we find that something unexpected has happened, and we're looking at the best UX to expose that to customers soon.