I don't think I ever read the Challengers of the Unknown as a kid. This would be because I never saw them, since their run was pretty much over before I started reading comics, and then when revived I had other interests. I bought the first Showcase Presents of the Challengers originally adventures mostly out of curiosity than a sense of "I so need to read this and found out how some stories ended!"
Since I've discovered them now, I've really enjoyed reading their adventures. Jack Kirby is credited with creating them, more or less, and they really are the prototype for the Fantastic Four. It's pretty easy to see, though they never had the emotional content that Stan brought to the FF. That's a story for another day.
Anyway,the first several stories are drawn and written by Jack Kirby and some of his friends He left after issue #9, and Len Brown took over the art chores (he also did some art for T.H.U.N.D.E.R. Agents). What is most interesting about issue #11 is the painted cover he did for DC. This is far more reminiscent of Gold Key or Dell comics back then. I like this cover a lot, more than the actual story. DC Comics seemed to love having word balloons on their covers... all the time. It usually detracts from the actual cover. Here we have some weird alien on a flying dinosaur/horse hybrid, and it's shooting laser rays at the Challs. There's no need for an words at all from our heroes.
It was nice to see the Challs in the Brave and the Bold cartoon They would have made a very cool H-B cartoon in the 60s.