I've only had very minimal issues with S3... one of the things thats holding me back from switching to a cheaper provider from Amazon is if its not broke, don't fix it.
Individual blob access is not very reliable on S3. In a previous company uploading big-ish (1-10000MB) files was part of the service. We would see failing uploads or slow writes all the time. Make sure to queue and wrap your upload script with retries.