If you consider the variables:
80 gigs of data to be uploaded - how long will that take with your Internet connecttion?
and then, in the case of a problem
80 gigs of data to be downloaded - how long will that take with you Internet connection
(and that's assuming that the Cloud service doesn't share or limit the download speed in any way, but, for instance, I've seen DropBox output at 10kb/sec... we're talking weeks to download 80 gigs of data at that speed.)
Then there's the issue of disk formats - will the service store the Library on an appropriately formatted disk, vital if you're doing incremental back ups - or will the Library risk being corrupted on restore?
I think we’re a very long way from a viable Cloud back up of an Aperture Library. Best advice for an off-site back up: copy the library to a portable HD and store it at a relative's house.
I have a 290GB Aperture Library backed-up to Amazon S3 via Arq ( haystacksoftware.com ).
It is a nice solution. The Arq software will back up your stuff to S3 or Glacier.
As Terence Devlin pointed out, you will want plenty of bandwidth to get your initial upload complete. How happy you are with it on an on-going basis will depend, in part, on how many images (and how large) you need to upload each day/week/etc. I use a Nikon D800, so my backups can take a few hours - might be intolerable for a wedding photographer (i'm just a hobbyist).
In any case, I would suggest also having local backup (both for its own sake, and especially to protect those images that have not yet been uploaded to the cloud).