My only complaint with the the magnesium block is the bulk. I ran one through a tile saw leaving just a few mm of magnesium next to the ferro rod and started carrying it much more often, leading me to use it much more often.

Magnesium shavings, in and of themselves, leave quite a bit to be desired as tinder...they do blow away and burn very quickly. Where they really stand out is when they're added to other tinder. And, as somebody has posted before, one can always scrape magnesium shavings / filings on to the sticky side of a piece of duct tape.

If somebody is going to cut down a magnesium block with a water-cooled saw, do so very slowly. You don't want the magnesium to heat up since burning magnesium and water is a death wish.
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(posting this as someone that has unintentionally done a bunch of stupid stuff in the past and will again...)