I'd imagine they rely on the fact that most people's uplinks are very limited. For example, here in Ireland the 24Mb package offered by most DSL providers has just a 1Mb uplink.
My own DSL line has a 384kB uplink which is the fastest my line will handle according to my ISP and an upload typically transfers at 38KB/s. Based on this speed, it would take 163 days of non-stop uploading to fill the 512GB capacity.
On the other hand, assuming their service keeps up for years to come, it would be good for incremental backups, such as backing up photos, documents and video clips as you create them, which could easily be uploaded overnight each day.