In one sense, yeah, it's way too easy to pick on movie serials like Superman. They were made quickly and cheaply, designed to be seen in 15-minute increments a week apart. So the occasional changes between the ending of one chapter and another, which seem so glaring when you're watching several episodes in a row on DVD, would have been missed by most of the movie-going public. The cheap special effects and repetitive nature of the plot are similarly easy to fault, but also completely understandable.
For what it is, there's a lot that's good in Superman. The plot, while repetitive, actually presents a broad range of perilous situations for its cliffhangers. Unlike Batman and Robin, which would be made by pretty much the same team--same producer, one of the same directors, mostly the same writing team, and even some shared cast--the car going off a cliff gag is only used once. Some of the super strength gags work well, and the animation, while clearly unrealistic, does lend a dynamic excitement to some scenes.
One big problem I have with the serial is with Superman himself. Kirk Alyn makes for a pretty good Clark Kent, but he just doesn't sell the superness of Superman. He too often comes across as good-natured, but stupid, especially in the final few chapters. And his dance training is all too evident, so that this Superman is suspiciously graceful. The stock shot of Superman ducking behind the filing cabinet to change clothes, for example, includes this weird throwaway moment where he arches his back and points his toe, a dainty little fillip that is weirdly distracting.
It's just not super.
The rest of the cast isn't bad, though. Noel Neill brings a lot of charm to the part of Lois Lane, even though she seems strangely unfazed by the dangers she is exposed to in nearly every episode. And Tommy Bond makes for a pretty good Jimmy Olsen, a scrappy kid who takes on thugs even though he has no super powers. Likewise, Pierre Watkin does an excellent Perry White and Carol Forman makes a pretty good villainess (though we never get any real idea of her true identity, history, or ultimate goals).
The most interesting thing, perhaps, is that, unlike the Batman serials, we of a certain generation instantly recognize the members of Superman's extended cast and understand what role each one is supposed to fill. There's none of the weirdness of "Where's Commissioner Gordon? Where's the Batmobile? Why does Batman keep talking about getting orders from the government?"
And to hammer this point home like a record that's been broken as badly as the one Superman repairs in the final chapter of the serial, it's due to the radio series. The characters in Superman are modeled very strongly on the characters from the long-running radio serial. The cast will remain unchanged in the next movie serial, and though the roles will be recast in the TV series a few years in the future, their portrayals will still hew very closely to the radio show.
The point is, the character that lives in the popular imagination as "Superman" is not the character from the comic books, by and large. It's the character from the radio show adapted from the comic that was mostly responsible for everything that came after.
Granted, this was more true in my youth than now. Nerd culture has begun to penetrate into wider consciousness, so that elements of the comic-book Superman, like red kryptonite and the Bottle City of Kandor--are now more widely familiar. Still, even the comic book Superman was greatly influenced by the popularity of the radio show.
However, one big element was missing from the first serial. The Spider Lady, while a decent serial master villain, was not a member of Superman's Rogue's Gallery. That shortcoming would be remedied in the follow-up serial, Atom Man vs. Superman. See you in four weeks (Nov. 1, a very special day) for that one.
UPDATE: Oh yeah, and just because I think it's funny, there's also this...
UPDATE 2: Marc Carlson in the comments mentions that he's not sure one can hide behind a cabinet "manfully." And I acknowledge that this may be true. But that doesn't mean you have to do this:
And that's what it calls to mind when he grabs the cabinet and leans back. Seriously.