I use it, it's fine as a "third place." Make sure you set the tier of the storage as the default Standard is fairly pricey. I use Standard-IA (Infrequently Accessed), which is still hot-readable but with a high retrieval charge—perfect for what should normally be write-only. A bucket policy ages out backups > 1 year. Encrypt your backups before you write them, of course.
There's nothing wrong with Backblaze et al. either. They may give much lower costs for bulk data. I only back up a few dozen GB so it's not worth opening another vendor relationship for me.
Somebody else suggested "hard drives at a relative's house." What's more likely, you lose S3 access—or your relative's house burns down / floods / is robbed / their kid yoinks a drive to use at school / any number of other disasters? Also, a backup should be as easy to do as possible—having to manually shuttle a drive back and forth from a remote location is probably not the best, unless you are extremely disciplined.
Assuming you have a local backup too, rsync.net is an interesting option. It even supports ZFS snapshots natively if you're using a NAS. In case of ransomware it's useful to have access to older snapshots.
Myself, I keep everything on my cheap surplus NAS (2 drive redundancy), snapshot regularly, and occasionally sync to an airgapped drive the next town over. I keep a cold copy at my parents house, but that's last-resort.
The important bit is that I've successfully trialed recovering from a total loss of my NAS and house, in case that ever actually happens :)
Frustratingly, the Backblaze personal backup client doesn't support Linux, because they don't want people to abuse the personal backup service for their server, NAS, etc.
A good third option is at the office (remember those?). Tends to be a little more secure than a residential building. Before the human malware I'd bring home the disk stashed in my desk drawer on Thursday, sync the data at home, then re-stash the drive on Friday morning. Good-enough solution for my comically slow upstream bandwidth at home.
(Worried about a physical hard drive getting lost or damaged...)