New Zealand, 1994 year:
Here, is one issue, that I will give little information, and probably some specialist will correct me some way, but my intention is not to give detailed description about this stamp.
I just examined my album, and saw that this stamp have some kind of "beauty", even that on "first sight" it is not so attractive!
Like we know, it is 1994 year, 18 Feb. issue of "Mount Cook" definitive.
Many of us will say, that it is no need to print 20$ face value stamp, except if Post made it intentionally ta "grab" some cash from collectors around.
Why I think, that it is worth to be included in every collection?
First, it is high value, that not every person will have budget to spend on, that mean, who have it - have some kind of "advance" over other, in collection value. Second, in used condition it will be more difficult to have it in collection, because not so many samples are actually used on any documents or parcels. And in mint condition, my professional opinion is, that because of printing, paper, glue - will be even more hard to keep it inside album.
I see, after 10 year in my album, that my two samples start to be some way "old", if it is chemical process of glue and color I don't know, maybe gold foil/color applied up/down also made some "feeling" of "old" stamp.
And if You will have in Your collection also one mint, that have some "error" (left stamp), don't "jump" of luck! Examine this left stamp, more closely, and You will see that it actually have this color variety of "damaged zero" because of printing process and not of "error".
This issue is printed in recess printing technique , where printed field You can feel under Your fingers (like on US$ bill), but it give very big "hard life" for printing operator, because of dry-time for color, if it is not dry enough, it will stick together (very softly) stamp - and when you separate it, You will even not notice that two stamp was stickied together, but color part will flee with other stamp, and You will think, that error in printing is discovered!
As is is - I think, that it is worth to keep it in collection, and used on document, will be worth premium.
From world catalog, I see that it is valued around 25.00 US$, and used sample have better value of mint. On New Zealand soil, it will be valued around 30.00 - 35.00 NZ$ (I think).