The other alternatives for us would involve moving objects out of S3 to some other object storage services, which would generate a lot of network traffic, or keep the objects on more expensive storage.
It's great for objects that are already in AWS, like in S3. Moving objects from S3 to Glacier is very simple.
It would be difficult to consider Glacier outside of other AWS services. It's unlikely that someone would use AWS just for Glacier. It's great as part of the AWS ecosystem, but shouldn't really be deployed as a stand alone product.