Sometimes when people translate Uni, they would say sea urchin roe, it is not really correct. It is the organs that produce the roe, the gonad. It sounds kind of gross but I think it is really tasty. (Maybe that is why the restaurant name it sea urchin but not sea urchin gonad)
I have heard that the best uni is red in colour, with orange and yellow as the 2nd tier and 3rd tier. Luckily it all taste good when it is fresh. When it is not, it taste rather disgusting like a rotten egg. Sea urchin lovers beware of your cholesterol level too, it is realy high in cholesterol and fat and you should desist after 2 or 3 pieces (probably you can't afford it anyways, it cost 3-4 dollars for apiece)
Wonder taste and aftertaste, buttery and fatty. Also love-it-or-hate-it food. Not recommended for first time sushi-eater or people who don't like fishiness. But it is my favourite seafood so far. Applications of sea urchin includes: chirashi(sashimi bowl), nigiri sushi, sashimi served with cucumber or seaweed, fried rice, steam egg with uni, many chef creation rolls etc..... Let me know if you can think of more.