Borgbase had a week long (IIRC) outage due to a failed attempt to add new drives to an array. As far as I know they never published a post-mortem on this and have never discussed how they're going to improve their disaster recovery so it can't happen again. It's difficult to recommend when they could leave you without working backups for an entire week.
Also you can back up to the hard drive under your friend's bed, and they can back up to the hard drive under your bed.
If you're even slightly technical, or have a friend who is, I'd recommend both of you buying the cheapest Kirkwood NASes you can find on ebay, throwing Debian on them, and becoming each other's backup buddies.