Are you specifically looking into offsite backup? If not, I'd just buy an external drive and back up to that. Faster, cheaper (per gigabyte and bandwidth if these apply) and more convenient.
Online backup is useful for so many reason:
No lugging around a backup drive if you use a laptop. (Yes there are wireless alternatives, but I change location very often between home, uni and campus, so if you're always on the move physical backups can very quickly fall behind)
Can set it to backup as often as you like, automatically, so do not need to remember to do anything.
Can restore files from any location.
Safe from theft/fire/natural disasters.
Keeps a copy of different versions of files so they can be rolled back.
Private encryption key so your files stay private.
But then there are disadvantages too...
Subscription service so you're always paying for it.
Then the main criticism - limited by your network speed. Uploading large files initially can take ages. But once that's completed incremental backups only take seconds so I don't find it a problem.
As well as that recovering quickly from a disaster can take a while if your download speed is slow.
So it's very much a personal choice really, there are reasons for and against. I personally think it's a very useful service!