As always, remember to do a restore to prove your backup is an actual backup.
Even at floppy speed, copying the 400MB wasn't that long, and it's lovely to hear the cute drive noise (for retro lovers).
It turned out to be totally fine, as far as I could tell. None of the early data had SHA/MD5 sums (I added this later), so there's no guarantee the earlier discs weren't somewhat corrupted, but they were definitely readable.
Back then, I had assumed they wouldn't have that long of a life, but as you seem to have found, and I discovered, once written, optical media seems to have a very long shelf life.
They are essential carving physical holes into a stone like substrate.
The verification tests are really quite impressive: http://www.esystor.com/images/China_Lake_Full_Report.pdf
Hosted backups are likely at least as reliable as anything you can do. Right?
PCI/IDE/SATA interfaces are so ubiquitous that even decades from now I fully expect you to be able to use them. If nothing else, via some kind of adapter.