Let me preface this review by saying the Wanderer is not a book that has held up well over time, and the most common question asked in a lot of the reviews you can read online is: "How did this book win a Hugo?" Never a good sign. The book has a lot of problems, the action jumps around too much, the characters are poorly developed, the writing in spots borders on unreadable, and the treatment of women in the story is emberassing. It's very curious that a book like this can come out of an author as famous a Leiber, one of the premier authors of the 50s and 60s, and one of the founding authors of the Sword and Sorcery genre.
Leiber is probably most famous for his 'Fafhrd and the Grey Mouse' series, I have not read it, and have no idea how to pronounce it. Leiber was one of the most prolific writers of the early sci-fi era, winning awards in Science Fiction, Horror, and Fantasy. He has over two dozen collections of short stories, and several novels as well. Sci-fi wasn't a very profitable venture in the early days, and those authors who made a living at it usually did so through quantity not quality. It was not uncommon to see authors publish two or three novels a year along with a couple dozen short stories, writing at this pace there will always be a few duds that leak through (see Silverberg, Robert). That provides a little context for why this novel is so difficult, but not really an excuse.
'The Wanderer' is probably most famous today for kicking off the sub-genre of disaster fiction. The novel describes the arrival of a new planet into our solar system and the havoc it wreaks as it devours the moon. Leiber bounces back and forth between the 'soft' science fiction of describing the effect it has on people, and the 'hard' sci-fi of describing the science involved with what is happening. The novel might better have been served by focusing on one or the other. Though the disaster novel has kind of faded away today to be replaced by the disaster movie (see: '2012') for awhile there disaster novels were a mainstay of both sci-fi and mainstream fiction. John Brunner took some inspiration from this novel in his work, as did John Barnes in his novel 'Mother of Storms.'
This novel seems to be another example of the idea for a novel winning the Hugo despite the writing. There is no denying that the idea for this novel is a grand one, the idea of a universe filled to the brim with life is more than interesting, and the conceit of a rogue planet on the run from the authorities is more than enough to inspire any writer. But, there is also no denying that this might be the most poorly written novel to ever win a Hugo. While there are some well described scenes in the book, the destruction of the moon and Don's escape through the middle of it for one, there are countless other examples of poorly developed characters and meandering plotlines. Other authors have used multiple perspectives to give the reader a general sense of a large community, but Leiber does a poor job here, jumping back and forth from settings so often that the reader is often left confused about what is happening or what character is doing what.
Women are poorly served in most early science fiction, regulated to damsels in distress most often or sex symbols as a back-up. Leiber does even worse here, where a woman's only choice seems to be between manipulative fool and crazy lesbian with a desire for death-sex (you heard me). While the novel is probably the first (and last) instance of a man having sex with a humanoid cat, it does nothing to serve the novel, and feels more like Leiber exercising a fantasy that creeps most people out. Sexism is prevalent, like I said, in most early science fiction and to be expected, but examples like this drag an already troubled book even further down. Sex becomes more common in sci-fi as we move into the early seventies, but it was already here to stay by 1965. While no one would expect equality of the sexes in 1965 you should remember that this book was published after Heinlein's 'Stranger in a Strange Land' and Dick's 'The Man in the High Castle,' two novels that took at least a slightly more mature and balanced look at the sexes.
It's tough to say something about 'The Wanderer' that hasn't already been written, it's a novel that has large defenders and just as large detractors. Marc Goldstein in his review of the book seemed to believe that Leiber intentionally avoided characterization in an attempt to support the story's underlying pessimism and the small impact humanity can have on the final outcome. This seems a little like saying that Leiber intentionally wrote a bad story, which is hard to believe or understand. More often there are reviews like Sam Jordison's who ask how this book could be nominated for any award. The answer to that is tough to say, but often, like the Nobel Prize or any major award, those giving out the award are actually looking at the entire body of work for an author, or their importance throughout the genre rather than looking at the specific work the author was nominated for. That seems like more of what we're dealing with here. Leiber wrote a poorly executed disaster novel that had a very promising premise and a few thought provoking ideas, if it had been another author writing it maybe it would have recieved just a nomination, since it was Leiber it won the award.
(Here are links to the two reviews I mentioned if you want to see them:
Jordison's 'Bad' review: Here
Goldstein's 'Good' review: There
These are good examples of some other takes on the material).