This was one of my all-time most anticipated movies. I had actually picked up the first issue of the comic more or less by accident (it was in the same issue with a new Steve Ditko story that is what caught my attention), but instantly became a huge fan (as my avatar may suggest).
I knew it had the potential for a great adventure picture, but I knew a lot would have to be changed: even without naming him, having Doc Savage as the inventor of the jetpack would mean less than nothing to a mass movie audience. And "Betty" was certainly not going to be a worldly, spoiled brat (based visually on Bettie Page) enraging Cliff by posing for a "cheesecake" (for that era, read: porn) photographer with whom Cliff believes she's having an affair. Which is just one example of the book's often dark tone, that, again, Disney was certainly going to jettison for a mass audience.
And I think the film managed the changes pretty well.
Making the source of Cliff's jealousy a movie star was reasonable, and as others have noted, basing him on Errol Flynn's supposed Nazi sympathies was the perfect touch.
And since, in the book, Peavy mistakenly "deduces" that the mystery inventor is Howard Hughes, that was an ideal transition.
I thought Campbell was perfectly cast, physically, and not too bad in the role. Connolly, of course, was gorgeous (maybe THE single aspect of the film I was most looking forward to!), but they could never seem to decide just how bright Jenny was supposed to be.
Some nice period touches (I loved W.C. Fields and Rondo Hatton-- they took his character from Stevens' second Rocketeer story), and most of the supporting cast (particularly Dalton, O'Quinn, Arkin, Lauter, and Polito) were golden. And the Nazi film is genius.
But then there's the stuff they didn't get right, and as others have alluded, it was the most important part of the film: there's just not much action, and what there is is only middlingly well done. There's not a ton of action in the comic, actually, but it's gorgeously rendered, and the "running time" of the original story is actually short enough that it doesn't feel like it goes on forever between action scenes. And while the period detail is nice, it's less striking seeing it in a movie (where that kind of professional skill isn't all that unusual) than it had been on a comics page.
I agree that they probably did the best they could with their budget, but that's not the kind of discussion I want to have with myself when I'm sitting in a theatre: I want to be transported, and all too often I wasn't.
Also, while I'm willing to concede that others may take to Paul Sorvino more than I do (I don't think he's much of an actor), his subplot was just too much. The film needs better focus, and balancing Cliff and Jenny, Cliff and Peavy, the mob, Dalton, the FBI, Hughes... it's difficult a task for any director (and editor), and one that just seem to be beyond Johnston's capabilities.
Someone called it "plodding", and that's dead on: it never has the narrative drive toward a conclusion; it's like we're constantly waiting for the movie's diverse elements to catch up with each other, instead of moving forward. For instance, while I liked seeing W.C. Fields, that nightclub scene is just one of many that is probably twice as long as it needs to be.
It has some great visual moments (Lindy's test of the jetpack is funny, and beautifully shot), but I've had the DVD sitting around for years and never felt motivated enough to put it in (though I did rewatch most of it on cable one time years ago, and my reaction was the same as it had been at the movies).
ETA:
A year or so before the film came out, my wife got me this t-shirt (the same design as my av) for Father's Day:
I wore it to Disneyland the summer the film came out, and got some interesting looks.
BTW, the model for Betty was Stevens' wife at the time, actress Brinke Stevens, who complained in an interview about the amount of time she had to spend lying across a sofa, bound and gagged, while she modeled for the picture.
