prism : "(just see your broadband bill when you've tried backing up 50GB on line)"
I can think of a couple of ISPs (AAISP for one, Zen perhaps the other) which do not count any upstream data in their monthly "quota". I don't know which "Fair Usage" ISPs would be unhappy with 50 GB, but the ISP I use doesn't count any traffic (upload or download) between midnight and 08:00 (01:00 to 07:00, I think, if you have higher speed via Fibre)
As for low upload speed, the initial backup may take a few days, but for peace of mind, that's not going to be a problem, and afterwards, any backup software should do incremental backups to only upload items which are new or have changed.
"or it doesn't actually work."
Clearly one needs to check backups, from time to time, and any reports generated by backup software, which would point to success of the process. I would probably suggest online backup in addition to any routine 'copy and remove from the site' routines for a small business, at least where invoices, quotes, and other correspondence is primarily printed and posted to customers, but even if items are sent by e-mail, it is easy to have a all new items backed up online, so even if there was some major problem (flooding in an office block from floor above, for example) which killed all the computer equipment, if there's software checking for new or altered documents "on the fly" then something which was saved to disk 3 minutes before the flood/ disaster, should still have a chance to have been backed up in time! Easy!
Yes, external drives / memory cards/ sticks can be useful, but online storage can have a place too.