I've been thinking about buying some to replace some old CDs, but have heard terrible things about the audio quality. Is the sound quality actually that bad or is it just audiophiles mouthing off?

I have the first seven albums remastered, and the sound quality is fine.

EDIT: There's another incentive to get the remasters: the updated booklets and the enhanced video content are nice.

The only versions I've heard are the ones I own, four remasters and the original of Piece of Mind.

If anything, PoM is the worst-sounding of them. It feels very flat, dull, and empty compared to the others. I have no idea, though, if this is just how PoM works. But yeah, the remasters sound fine. I'd certainly be complaining if they didn't.

Well, I own Killers and Somewhere In Time - both the "enhanced" editions aka the remastered ones. Both of them sound spectacular, especially Somewhere In Time. Everything you could hope in a production boost is pretty much there. If anything, the bass gets a bit drowned out, but that's probably because I blast mine to the max.

I think read somewhere that the original Iron Maiden cds was just the vinyl track put on a disc, and therefore sounding worse. But I could be wrong, because I don't really know much about that stuff.

That was the case with 80ies albums. I own a couple of Anthrax and Testament and it was written that they were analog tracks. That doesn't mean it sounds worse (in fact, usually better), but with any band with an ounce of popularity, their material gets digitally remastered.

It isn't really a problem. I recently bought a compilation CD, all remastered tracks. Compared to my originals there isn't really a serious difference. Besides, the song quality can't be ruined by such minor things.