The success and failure of something always depends on how well its appeal translates beyond those already predisposed to like it. The Rocketeer, for instance, was designed to appeal to people nostalgic for old Hollywood serials. It met an enthusiastic response from critics, and gained enough exposure that it joined the rush to find movie vehicles to complement 1989's successful Batman reboot. And it was also one of the many, many failures to do so.
Now, such failures aren't always because of the actual quality of the product. Success does not translate into quality anymore than failure translates to a lack thereof. The failure of 2011's Green Lantern but the success of 2008's Iron Man says more about capturing the zeitgeist than anything. The latter' best trick was pulling Nick Fury and the promise of the Avengers, creating the idea of being a part of something, out of its hat. The character himself was as culturally meaningless as Green Lantern, whose movie existed in an equally meaningless vacuum.
Dave Stevens created an ode to the old serials in the wake of Star Wars making them relevant again. But Star Wars, and later, Indiana Jones, did it by making the concept new and vital again. Rocketeer is virtually exactly like the old serials. That's why the movie failed whereas the character in general still exists in sporadic new material (the same can be said for other cinematic failures of the time, including the Phantom, the Shadow, and even Dick Tracy).
The original comic makes all this clear. If you're nostalgic for the stuff, you'll probably love it. The effusive praise from Harlan Ellison will ring true to you. If you're not, it reads like hokum, and Ellison sounds like he's blowing so much hype. If one thing's clear, it's that the design of Rocketeer is the one truly timeless element of the story; everything else pails in comparison, just plain doesn't hold up.
The absolute strangest part of the whole thing is that Stevens takes repeated jabs at Hollywood, even though his inspiration clearly comes from Hollywood. The movie feels more natural because the material is in its element; the comic becomes more limited in its appeal the greater the distance from when it was created. Ellison references other comics featuring nostalgic acts like this, but they're already lost to history nearly forty years later, the way the serials were when George Lucas began raiding them for inspiration.
So I rate this as less than its reputation would suggest because it's not as good as it ought to be. A thing for nostalgics.