Other reviewers speak much more eloquently about this book than I can. So I'll try to be kind of brief in my observations about this book.
1) You have a donut and a donut hole. The story of Ahab and Moby Dick is the donut hole. Herman Melville, clearly wishing to write a non-fiction account of the mid-19th century whaling industry and using this book as a vehicle for that, is the donut. So don't go into Moby Dick thinking this enormous book is all about mad Ahab's quest to get revenge upon The White Whale, because it is most decidedly not all that story. Ahab and Moby Dick are only a tiny, albeit majestic, portion of the whole.
2) I had to give this book five stars. The two stories I mentioned above, when put together, result in kind of a hot mess. They don't always come together beautifully or with great results, but together they also have a certain je ne sais quoi that shouldn't work, but somehow manages to do so. Individually, however, the two stories are both undeniably works of literary art, and they are absolutely labors of love. Individually they are amazing, and I don't have it in me to devalue the awe-inspiring aspects of this story just because Melville probably would have done better to write two separate books instead of trying to cram all of his passion into this one book. The book takes a lot of willpower to finish, but there is just layer after layer of information (some fascinating, some not), wonderful language and writing, deep study into the human condition, fascinating philosphy, sharp social commentary, and breath-taking storytelling here. This book is dripping in gems!
3) If you have to read Moby Dick, or choose to read Moby Dick, I would like to suggest that you listen to the audiobook version narrated by William Hootkins. His narration of this book won the 2006 Audie Award for Solo Narration, and I know I'd have never finished this book if William Hootkins hadn't been reading it to me. I'd have never made it past chapter 10, if that. I think I'd have gotten even more out of this book if I had read along as Hootkins was reading to me, but I listened to the book at work, and they'd probably frown on me reading instead of working. If you find Moby Dick somewhat intimidating or impenetrable, find Hootkins's audiobook, and listen or follow along. He makes it totally accessible. He even somehow makes the boring parts roll along less, um... slothfully.
In the end, I was very pleasantly surprised by this book. I've always liked Herman Melville, and I'm so glad that Moby Dick didn't put me off of him. It was a glorious story.