Let's all just take a look at that picture from the Macy's Thanksgiving Day parade in 1940. Superman debuted in spring of 1938. That means...within 2.5 years, he was popular enough for this image. Nowadays, that wouldn't mean much, since media have made saturation happen much more quickly. But in 1940, it was much more impressive.
This leads into some further thoughts about Superman and Gladiator.
I suggested a few days ago that the essential optimism of Superman ensured his popularity, while the pessimism of Wylie's Gladiator contributed to its relative obscurity.
But that can't be the whole story. Saturation is also important. Now, I don't know how books were sold in the 1930's, but I bet they weren't as readily available as comic books. (Books at the end of the 19th century were usually sold by subscription--that is, door to door). Putting comic books on the shelves of markets and drug stores, newspaper stands and the like ensured that lots of people (not just kids) saw them. Gladiator wasn't nearly so visible.
By the way, Siskoid has a blog entry about Superman balloons.