First, from the public health angle: Does it have mercury, everyone wants to know about any seafood, quickly followed by, I don't want nasty parasites.
Mercury, like many other toxins, bioaccumulates. Animals don't break it down or digest it, so it gets concentrated up the food chain. Suppose a single sardine has a small amount of mercury. A squid that eats a hundred sardines will accumulate a hundred sardines' worth of mercury. A tuna that eats a hundred squid gets ten thousand sardines' worth of mercury, and a shark that eats a hundred tuna . . . well, you get the idea.
The moral is: eat lower on the food chain to avoid mercury. Humboldt squid, despite their deserved reputation as voracious predators, are actually fairly low on the food chain, since they mostly eat small stuff. Thus, mercury isn't a concern.
And neither are parasites. Neither dicyemids, shark tapeworms, nor anything else living in squid is a very effective human parasite. Like parasites of other exclusively marine species, they haven't evolved to infect terrestrial mammals, so they don't have a good strategy for doing so. It's the freshwater parasites that have evolved alongside humans and figured out how to infect us, so freshwater fish are a greater cause for parasitic concern.
That said, some marine parasites can actually make you pretty uncomfortable while they're blundering around being ineffective infectors. (Some are included in this nice list!) But even considering that, Humboldts are still pretty clean.
So from the perspective of human health, Humboldt squid steaks get a solid thumbs-up. (But don't eat them up off the beach, please. That's just nasty.) Tomorrow I'll consider fishery health . . .