Wonderful nostalgic stuff. It brings back school summer holidays, watching this particular story or the other Buster Crabbe Flash Gordons (Trip To Mars, Conquers The Universe) or King Of The Rocket Men (which must have been an inspiration for Stan Lee, Larry Lieber, Jack Kirby and Don Heck when they created Iron Man).
Despite the black and white, if you can get one of the better prints, it holds up remarkably well, in that it's still very entertaining. OK, Buster Crabbe's stunt "double" is a rather podgy bloke in a blonde wig, and some of the acting is somewhat stiff, but it's all worth it for Charles Middleton chewing the scenery as Ming The Merciless.
Looking back (and having seen it more recently), none of the other Republic/RKO serials can really hold a candle to this one. For one, it had a massive budget, comparatively, and it shows.
As I said, still eminently watchable and almost as much fun as the Dino DeLaurentis version from the late 70's.
I await Taika Waititi's take with great interest.