I've backed up a load of photos (mostly raw, 5 discs so far) onto 100GB M-discs. They seemed like a good choice for long term archival storage - they don't need to be kept in ideal conditions - humidity isn't supposed to be a problem, and leaving them out in the sun is apparently fine! Plus drives for reading them are cheap and readily available.

The discs themselves are quite expensive in terms of cost/TB - somewhere around 50-60 GBP from what I've found.

I have more confidence in their ability to not lose data if left on a shelf and forgotten about for several years compared to hard drives. I have the same data stored on a ZFS pool too.