Here's the basic scoop on M-Disc:
As a long term archival solution, it is currently the best media available (on the assumption the disks are as durable as claimed, and testing, even by the U.S. Navy seem to uphold that premise). Google M-Disc torture testing, there's a lot of tests having been done on them.
Regular CD's, DVD's and Blu-Ray disks that we're all familiar with use an organic dye as the burnable portion of the disk to record the data. That organic dye degrades. It literally begins to degrade the second you burn a disk (some argue that the dye begins to degrade the moment the disk is manufactured). Years ago we were all told that recordable CD's and DVD's would be good for 100 years...but the truth is, they aren't. With good grade media and careful storage they are reliable for 10-20 years. Careless storage (like thrown around on a desk, layed in the open and exposed to sunlight, etc) 2-5 years. Sure, some folks have disks that are much older and still work (even I do), but you can't rely on that, it's a crap shoot and those disks will continue to degrade and eventually be useless.
M-Disc changes all this. The burnable portion of the disk is made of a mineral substance (some call them "rock" discs). This substrate is an inert, inorganic layer that does not degrade over time. They claim 1000 year life span, and the only reason it is 1000 years is because they will oxydize in that time due to the polycarbonate plastic layers...not because of the mineral data substrate. Technically, the etched mineral layer has an indefinite lifespan.
To use an M-Disc does require a burner made for it. The laser is more powerful than a standard burner to be able to etch the mineral data layer. They are readable in regular drives though, although not 100% (then again, that same quandry exists even with the old style dye based disks).
In a sort of funny twist, you can think of M-Disc as literally being etched in stone, much like the old Flintstones cartoon where they chiseled out information on stone tablets.
I would not consider M-Discs a great backup solution (in terms of backing up systems, etc) because as mentioned, those backups are only good for a certain time frame (do you really want to keep a permanent system back up of your current I7-4590K based PC when in 5 years you're going to have a completely new system based on whatever is available then and whichever OS?).
M-Discs are, however a much better "archival" solution than current dye based disks. As a hobbiest photographer, I have terabytes of photos to store. I currently have a 4TB NAS on which they are all saved, but those are mechanical drives and they will fail. M-Disc, for me, is a great solution for a much more permanent and reliable archiving solution for my photos. I also understand that the Library of Congress is in a mad rush to get a solution for archival storage as well to replace the burned disks they have. I believe they are looking at M-Disc for that purpose.
Of course, M-Disc eventually will go the way of the old, original 1x CD's when newer, just as, or more permanent solutions get invented that hold 10 or 100x more data and are faster. Unfortunately, that is the nature of technology. No matter how great something is today, there will be bigger, better, faster solutions in the near future. So, the question is, do you invest in M-Disc now and use it (with the potential to have better solutions in the future, and the cost to upgrade to that), or get M-Disc and stick with it? M-Disc is a good long term solution now and will be viable as long as M-Disc media is available.
M-Disc media is still pricey. I saw a 10 pack of BDXL M-Disc media (50GB per disk) for around $65. That's at, or perhaps slightly over the per MB cost of modern mechanical 500GB hard drives, but...we're trading cost for long term archival storage fidelity. Right now, it's absolutely an affordable format for anyone wanting or needing reliable long term archiving of files and data. Hopefully, the media prices will come down a little as time goes on, at least as newer technology comes out.
I did find the following interesting article from 2013 about a possible new technology that could replace M-Disc. 360TB discs, based on crystaline recording substrates:
So, the bottom line: If you have data that you need or want to keep, basically the rest of your life, and you need to save it to media that will reliably archive it without a lot of worry as to whether it will be readable in 10, 20 or 50 years, M-Disc is a viable solution available now.
Good burners that support Blu-Ray BDXL M-Disc format are hitting the $60-$80 price range. With M-Disc media running around $6 per 50GB disk when you buy them in bulk packs (or cheaper single layer 25GB disks) it's not a huge hit to the wallet to ensure your important data is reliably archived.