I remember about a decade ago, my brother-in-law used a $100 specimen note as spending money. My question is, are specimen notes prized highly given their, I'm assuming, rarity? And second, should I feel bad that such a note was used and not saved? Thanks. That second question was mostly meant as a joke. Mostly.
In general, yes a specimen note is much more rare than a circulating note. But it would depend on the country and specific specimen - there are certainly specific specimens that are currently more common than the same circulating note.
If that was a current US $100 note, yes, you should feel bad, because such a specimen of American currency would be worth several hundred dollars, at least. If it was not US currency, see Dave M's reply above.
I'm wondering whether a specimen note is considered legitimate as currency and can be legally spent. Anyone have an opinion?
My *opinion* is that they would not be considered legitimate currency. But I don't have a source for that. Good question.
Serial numbers can help tell if the "Specimen" overprint is a fake. Older notes are all zeros, later one "ladder" serials like on these notes. As for the orange specimen overprint - red is an unstable ink color; just a little too much exposure to bright light, and it is no longer red.
Any idea when the changeover from all zeros occurred? FYI, zeros remained the industry standard for specimen stocks and bonds.