I saw this serial shortly after it was commercially released on VHS in the late 1990s, and have not seen it since. I do, however, remember enjoying it.
I must correct some misinformation in the post by blackcurtain about The Original Shadow. The writer infers that this Shadow serial predates the radio series, but this it totally false. While it is true that the original Shadow on radio was a narrator for another show, he was the central character of his own series starring Orson Welles by 1937. Welles left in 1938, but the show was already hugely popular, and actors William Johnstone, John Archer, Steve Courtleigh (for a whopping 6 episodes, all of which are lost), and Bret Morrison carried the series all the way to 1954. And so, by the time the serial came out in 1940, the official radio series of The Shadow that is still famous today had already been on the air for three years.
I will, however, agree that this portrayal of The Shadow was much more in likeness to the portrayal in the pulp magazines. The 3 movies from 1946 that starred Kane Richmond are also closer to the magazines than the radio version, except for the humorous elements. In fact, the only version of The Shadow that I've seen on film that actually became invisible, other than the 1994 Alec Baldwin movie, was the hideous black and white filmed version from the 1950s called Invisible Avenger. If someone were to try to torture me, they couldn't do much worse than to tie me up and lock me in a room with nothing but that DVD set to endless repeats.
But this version of The Shadow, while not the best, is still enjoyable. It's doubtful I'd want to watch it again at over four hours in length, but I did enjoy seeing it the first time.