It's hard do agree if this is mostly a work of science fiction or fantasy. I think "Lord Valentine's castle" is more in the fantasy realm, but Silverberg accomplished, in this acclaimed book, a great blend of both. The story, the names, the characters, all have a big resemblance to Tolkien's work, but Majipoor is a world that stands on its own.
The story is simple, a quest in fact: Valentine "wakes up" as a thirty-year-old man whose past is misty and pratically lost. In Majipoor, a giant planet, metal-poor, and home to more than 20 billion people of diverse races (humans and aliens), Valentine travels hundreds of thousands of miles, gathering interesting characters around him, trying to disclose who he was, and what has been stolen from him.
Many other reviewers state that Valentine's quest has little and simple difficulties, and that's right. His group has many problems along the way, but every time they resolve it quickly and thoroughly. I think the main feature Silverberg accomplished in this work is to present Majipoor and its people. The governing system, the grandness of scale, the unusual situations and most of the characters are really interesting, and the reader can't help but like them, in one way or another. But sometimes, due to the number of characters existing on the story, some of them disappear for many pages, only to come back again, in minor passages, as if the author suddenly remembered about them.
The only thing that doesn't fit is the lenght of the book. Suddenly, it's all over. Silverberg could have developed the last two chapters of the book in a more complete fashion, adding another couple of hundred pages, and the book wouldn't be too long.
Anyway, this is one of the best ideas I've come along regarding fantasy/sci-fi. Great reading.