It is a very difficult set to complete for all the reasons folks have stated, but it is also a satisfying set. I think I am the person Ken is referring to as needing only the Henry to complete my set. That card came to me as a result of a chance encounter with another collector (and NET54 Board member) standing next to me in Ken's booth a few years ago. So, yes, Henry is the stopper that keeps many people from completing the set. (By the way, I would add Lee Magee, another difficult, but not impossible card, to Frank's list of scarce 222s.)
I don't know that I would limit myself to not starting the set unless I had a Henry card in hand. I think it is a fun set to collect because of the quality of the photos. And now that prices for the cards have settled down a bit, the set can be put together with comparative ease. I am helping a friend of mine do that. Besides, what's the fun in collecting a set if you don't have to spend time looking for the elusive card or two or three that you need to complete it? Bob is finished with his Obak 150 subject backs, but I need six more cards to finish mine, and I enjoy the challenge of finding them.
Another issue regarding T222s is their fragility. Because they were printed on thin photographic stock, it is difficult to find them in Vg or above condition. So, if you are a condition freak, you could become frustrated. My Henry card is trimmed, and one of the most condition challenged cards in my set, but I was not going to pass up the chance to acquire him because I knew it would grade AUTH.
Finally, one of the most definitive sources of information on T222s is on Bill Cornell's website,
http://.t207.com/t222.shtml . Bill has done exhaustive research to fill in the complete story on the issue, and all of it is presented in informative, readable prose.
Here's my Henry. So start the set--you'll enjoy yourself.
Cheers, Mike