Uni is the Japanese term for what is commonly referred to as sea urchin roe but is actually the gonads of the sea urchin -- of which there are five, the only edible part of the little fellas, which like starfish and other echinoderms are pentaradially symmetric. Uni has a reputation for being a love it or hate it food, which I think is owed to the fact that so much of what is available in restaurants is previously frozen Grade C.
Urchin is highly perishable, and the flavor degrades quickly. It's eaten in most parts of the world where it's a local food -- cracked open and eaten straight from the shell with a little lemon in the Mediterranean and South America, served as sushi or sashimi in Japan, and Wikipedia (which was no help in checking my spelling of pentaradial) tells me that in the Orkney Islands it was once used like butter.
Grade A isn't all that much more expensive than Grade C: it's still less expensive per serving than steak, for instance. I think the perishability has a lot to do with uni's off-putting reputation -- you can really only depend on its quality at a restaurant that sells it quickly and never gives it time to sit there getting old.
A date and I had our first uni experiences this weekend. (Well, I had a sandwich Thursday so I had some sense of what flavors would go with the uni - no photo of the sandwich, but I spread uni and cream cheese on rye bread, much like that open-face salmon sandwich, with much of the same appeal.) It's definitely a strong flavor, but not in a bad way. It's oceanic but not fishy, sharp, kind of tangy, kind of ozonic. I don't really know what to compare it to other than itself. It reminds me of trying to describe scallops - once you've said they're sweet and shellfish, you're sort of out of words, without having said anything yet.
El Quinto Pino in NYC serves (served?) an uni panini with soy sauce and a sharp Korean mustard oil, which I think must be meant to emulate the soy/wasabi notes of the sushi experience. I liked the idea of the panini but went a different way with flavors:
The corn on the cob and fried chicken were partly in case we hated uni, and partly because it doesn't seem to me like a flavor you want to fill up on. The sandwich is uni, butter, brown rice miso, and some shredded shiso leaves which were totally lost in the mix but also not really necessary. The combination was rich but not overwhelming. Really good.
Today we had a pre-lunch salad using up the rest of the uni: radish, cucumber, and double-peeled fava beans with a vinaigrette that's just uni, barley miso, lemon juice, and olive oil, whisked together and then pressed through a sieve. I wasn't a hundred percent sure how well it would work, although cucumbers go with damn near any flavors, which is why I love them so much in the summer (and this is very typical of my summer salads -- cucumber, radish, dressing, very little else; in the fall and spring I'm more likely to use lettuce). But again, it was very nice. The other vinaigrette ingredients cut the uni just enough that you could still taste the cucumber, but that beachy oceanicness was definitely present.