Yes they're a really useful feature. You can use them to allow both downloads and uploads. The latter is particularly cool for creating scalable webapps that involve users uploading large content.

Rather than having them tie up your app server resources during what could be a long/slow upload, they upload directly to S3, and then your app downloads the object from S3 for processing. If your app is hosted on EC2 you can even have the entire process be bandwidth free as user-to-S3 uploads are free and S3-to-app-server downloads to EC2 are free. Just delete the object immediately after use or use a short lived object expiration for the entire bucket.

I've made a note to add an example of this to my next post on this topic.