Moby-Dick isn't so much a book as it is an experience.
Despite the presence of Ahab, Starbuck, Queequeg and the whale, Melville's magnum opus isn't about its characters. It isn't even about it's own story. Sure, on the surface it's about a bunch of guys on a boat chasing a whale, and led by their mad captain into sure death all for the sake of vengeance, but the actual movement of that plot makes up only about ten percent of this brick of a book. We get virtually no backstory on any of the characters, and almost no dialogue, in the traditional sense of the word. All that matters to Melville is the here and the now, and it matters to him in exhausting detail.
The book is basically split into three parts: 1) Ishmael (our narrator) meets Queequeg and together they board The Pequod, 2) Extremely detailed but weirdly and fantastically wrong about almost everything science-related (Dear Mr. Melville: WHALES ARE NOT FISH.) descriptions of whales and whaling life and the whaling process, and 3) The Pequod finally meets the White Whale and Ahab meets his doom. Of these three parts, #2 is by far the largest, and Ahab and his whale, by far the shortest. Yet, Ahab and the whale are what everyone remembers.
The result of all of this for me was that Moby-Dick ended up feeling more like an extended meditation on life than a novel. Melville's characters are just avatars through which the mysteries of the universe, and the vast incomprehensibilities, wonders, and dangers of the ocean channel themselves. This book isn't about mortals, it's about Gods.
So: while it was definitely worth reading once (despite it's bad reputation and extreme length, made worse by lack of plot), it's not a book I ever see myself reading again. There's just too much of it, and I do mean that in more than a literal sense.