The "roe" that is served raw as sushi or sashimi are actually the ovaries from the sea urchin. It's typically eaten with soy sauce and a touch of wasabi. If served as gunkan sushi you'll sometimes see a quail egg added on top. When served as sashimi, it's not uncommon to have it served on a shiso leaf, or a slice of cucumber, although many other items will match up well also.
When it's fresh you can enjoy a creaminess like flavored butter, often with a sweet and sometimes a nutty taste. It always has a fairly strong taste, and an ocean-like flavor can usually be tasted fairly easily.
The term uni means "sea chestnut" in Japanese, and the best varieties when fresh and firm will have a slightly nutty taste. But beware the un-fresh uni, which usually tastes bitter, and possibly even worse.
Uni is graded according to freshness, color and texture. Uni gets grades A, B and C - you can guess which is the best right? Bright yellow or gold colored nice firm uni is the grade A stuff, and is typically sweet tasting and light. Grade C uni is often off-color, or damaged.
There is no substitute for freshness with uni. Nama uni (fresh uni) which is removed from the living sea urchins to order is noticeably fresher than one or two day old uni. Uni tends to lose it's sweetness and texture a little more with each passing day.
Uni is healthy as well, in moderation of course. Uni is low in calories, with an average sized serving containing around 30 calories. And uni is rich in omega-3 fatty acids. Careful eaters will not top their uni with quail egg, as a typical serving already has a gram of protein, and the egg tends to mask the flavor of the uni.
Historically Japan has been the primary market for uni, often buying up major portions of the harvest from the United States and Korea. But demand has been rising for years in the US, and in several other parts of the world.
In surveys of sushi eaters, uni always ranks near the top for people's favorite AND near the top for people's least favorite sushi item. There seems to be a sexual bias as well, with men naming uni as a favorite more often, and women declaring uni as their least favorite more often than men, and in fact more often than any other sushi item. Here is an often cited survey of 5000 sushi eaters from japan-guide.com illustrating those preferences.