The episode I actually watched today is "Chapter 12: Doom of the Dictator," which is the last episode of the last serial. So I'll just address them all briefly.
I've found that watching the 1930's Flash Gordon serials has been not only enjoyable but extremely educational in understanding the history of SF films as a whole. For example, I've heard lots of people criticize George Lucas as being unoriginal, and now that I've seen Flash Gordon (and read Asimov's Foundation trilogy) I can understand why. The grand, flowing music, the wildly varied environments, the exotic characters, they're all here. There's even a city in the sky, lightbridges, beings that live in trees, the wiping dissolves from one scene to another, and of course that famous slanted crawl telling you the story so far. But Lucas put his own stamp on everything; to me it seems obvious that the similarities are not an attempt to rip off Flash Gordon but a loving tribute.
Flash Gordon: Space Soldiers, the first serial, was definitely my favorite. Flash Gordon's Trip to Mars, the second serial, was fun but felt a lot more simplistic. The world and characters of Mars felt a lot more two dimensional and lacked the complexity of the first serial.
Flash Gordon Conquers the Universe is much better, in that it returns the story back to Mongo and feels more like the first serial. The rock people were especially cool, with the effect of having their dialogue all dubbed in backwards to make it sound like an exotic language. (Again, a feel that Lucas duplicated with his aliens.)
Another thing about these old serials is they feel so much more innocent. Today we tear things apart and bitch about continuity... back in the 30's they didn't expect to get over-analyzed and just went with what they thought would make for a good afternoon's entertainment. (Conquers the Universe actually seems to ignore Trip to Mars completely.) Why, of course Dr. Zarkov knows the rock people's language and can figure out all of Mongo's technology-- he's a scientist! That's all you need to know! And we can just smile and laugh about it and forgive it because, well, it's old. We accept that it's a classic and we acknowledge they had different standards at the time. (I disagree with those that say we're "more sophisticated viewers" these days... it seems to me we're just more unforgiving.)
The only thing I personally have a hard time forgiving is the recasting of Princess Aura from Priscilla Lawson to Shirley Deane. Priscilla Lawson's Aura was a hot, busty, dark-haired self-reliant bad girl! Shirley Deane's Aura was a skinny, blonde, wimpy damsel in distress. Rich told me that the many casting changes between the serials was because the actors often had exclusive contracts to different studios, so it couldn't be helped. Ah well.
All in all, I really enjoyed these, and it makes me want to look into the classic serials for Buck Rogers, Crash Corrigan, Zombies of the Stratosphere, and so on. Not to mention the Filmation Flash Gordon cartoon. But I'm going to wait until I finish Macross Frontier and Clone Wars first!
My last word on Flash Gordon: the original serials squash the Sci-Fi Channel version like a grape.