What really concerns me about M-Discs is the multiple reports on Amazon.com that they don't have a handle on quality control. If one or more discs bought in a single pack fail right off the bat, how are we to believe that the ones that are written successfully really are going to last for a long time?

If your really important data, the stuff that you don't want forgotten afterwards, is relatively small (vs. mass market media where there's always more copies out there), I'd go with high quality CD-Rs in environmentally well controlled locations, unless MAM-A Gold DVD recordable media turned out to be a lot more reliable than we expected. Don't know about BD recordable, it's new enough and small enough compared to LTO tape, that it's not currently attractive to me. Of course, you can use several types of media, and you really need geographic dispersion as well.

Tape is fine, as long as you keep renewing it, buying new media and copying your stuff to it. Storing it with two or more high tier cloud providers might also be a good gambit.