I just wrote a piece about how we use Amazon S3 to manage the downloads of Mule Enterprise. Putting a mule on a cloud: one man's battle with Amazon S3 is now up on El Reg.
We had been managing and maintaining multiple archives on multiple servers. After a while, a server crash, a disk blowout, lack of memory or some other fiasco kept reminding us why we hate computers.
This brings us to reason one for liking S3: you can stop buying and maintaining tons of machines - at least for file serving over the internet.