Originally posted by cdd29 Are they that superior in IQ or just the novelty of being a little on the radioactive side?
That depends on what you mean by "superior in IQ". If all you want is sharpness with a DSLR then you're better off using modern lenses. But if you care about the character of a lens's rendering, which is something much more personal and much harder to define, then you might just find that thoriated (and non-thoriated) legacy lenses suit your tastes.
To try to boil some of the differences down to their essence:
Modern consumer grade lenses tend to give high contrast and bright, vibrant colours even in dull light that is actually low in contrast and flat in colour. Apparently that's what the market demands. Legacy lenses (I'm talking about Takumars in particular) tend to give a more naturalistic rendering that puts the onus on the photographer to find really good light to shoot in. Shoot in flat, dull light and you'll get flat, dull results.
Modern consumer grade lenses tend to produce results with high edge contrast, giving a superficially sharper appearance to shots taken with them. Legacy lenses tend not to have such high edge contrast, and instead give you more of what's usually called micro-contrast, resulting in photographs that might look less superficially sharp at first glance but have a more three-dimensional feel.
The best modern lenses, such as the Pentax Limited series, are much less prone to over-exaggerating colour and edge contrast than the consumer grade lenses, but of course they are also expensive. The major attraction of legacy lenses for me personally is that they give me a subtler, more naturalistic rendering than modern consumer grade lenses at a very low price.