Its usage here is a colloquial expression meaning, they're very good. The "darn" is one of a number of words used, including "damn" and they are generally used when one wants to be polite, instead of uttering a swear-word, which, regrettably, would probably actually be more common amongst adults. Or perhaps that's just the company I keep .
Yes, darn is used as an intensifier, so it can emphasize positive qualities as well as bad ones. I don't find this use surprising, or unusual; it seems to me common in AmE.
Note: The WR dictionary doesn't have this definition for darn, but the one they give for darned works for darn as well. Some of the synonyms they give can also be used both ways, but others are used only in negative contexts.
As a euphemism for 'damned', it's probably not much used, but I think it survives in its own right in a few fixed phrases. As well as your original phrase, I'd say: you're darn right it is; the whole darn thing. There might be one or two others.