I'd call S3 super-reliable rather than somewhat-reliable:
"Amazon S3 is designed to provide 99.999999999% durability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.000000001% of objects. For example, if you store 10,000 objects with Amazon S3, you can on average expect to incur a loss of a single object once every 10,000,000 years. In addition, Amazon S3 is designed to sustain the concurrent loss of data in two facilities."
It's slow as a result...but that's the trade-off you're looking for in a backup.
Yes, unless there is a billing problem with your account, someone is causing a problem with your account, etc. As an Amazon S3 and AWS user I would say they are fairly reliable overall - nowhere near super-reliable!
"Amazon S3 is designed to provide 99.999999999% durability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.000000001% of objects. For example, if you store 10,000 objects with Amazon S3, you can on average expect to incur a loss of a single object once every 10,000,000 years. In addition, Amazon S3 is designed to sustain the concurrent loss of data in two facilities."
It's slow as a result...but that's the trade-off you're looking for in a backup.
http://aws.amazon.com/s3/faqs/#How_durable_is_Amazon_S3