I’ve been there a couple of times, I think it is fairly impressive, but not to the point where it’s a complete mystery as to how he did it; it was hard work, I’m sure, but not impossible, and not requiring any “lost knowledge” of the “secrets of the pyramids”.
I flipped through some of Leedskalnin’s essays on electricity and magnetism, and, as Erroneous mentioned, his ideas on that were pretty out there. My overall impression of the guy after visiting the place was that he probably wasn’t a well educated man, but did have a good intuitive understanding of the mechanics necessary to build a place like Coral Castle, while also having invented his own wacked-out theories on how things actually work, like electricity and magnetism.
About whether people saw him work, I think it’s true that he was quite secretive about it; I believe he had a tall wall surrounding the site so no one could watch him work. I have seen the pictures of some of the tools he used, but I imagine there aren’t many, and I doubt he ever had any pictures taken while he was seriously working on the place.
Some of the blocks of coral are huge, the largest being close to 30 tons, I believe. There’s also the famous 9 ton piece of coral set in place as a rotating door, which is quite nice.
And about Billy Idol’s Sweet Sixteen, I do believe the song was, in fact, inspired by the story. The video was filmed there, and there were pictures of Billy hanging out at the Coral Castle.
Finally:
That’s the way I feel about these things, too. I think a lot of it comes from the average Joe thinking, “Hell, that was 3000 years ago, people was stupid back then, not smart like we are now; I don’t see how they could have done it without help from them space aliens (or whatever)”. Basically, I think it’s just an insult to the ancient Egyptians (or whatever group of people you’re ridiculing at the time). Which is not to say the pyramids (and other things) aren’t impressive, they’re damned impressive. But how did they do it? They were damned smart, and knew what the hell they were doing, simple as that.