Well, I finally broke down and slugged my way through it. (Actually, this was attempt number two.) I may be way off, but to me this is a novelist's novel, a literary effort best appreciated by literary students and not by one who reads purely for pleasure and intellectual or artistic gratification. To someone who, like me, just likes to read and has always been curious about this book, I would suggest choosing something else.
The novel defies standard evaluation (i.e., assignment of three stars, four stars, etc.) owing to its scope and the unique nature of its aspirations. The text takes up about 1,200 pages in conventional format. (My copy is 700 pages of microscopic font crammed onto the page.) Is there 1,200 pages worth of plot in the book? Absolutely not. 1,200 pages of philosophy? Doubtful. Still, as the story's narrator explains, a person's entire life can be told in two pages, or a thousand pages could describe a single event. The book is in part a study of time and its measure - it does not seek to develop in the same manner or pace as other novels. To tell this particular story in the way Mann wants it told, a great deal of pages are indeed needed. This notwithstanding, the book is best left, in my opinion, to those who really like to make a study of what they read. Mann himself suggested reading it twice. (The only problem with this approach is that it would take 18 years of one's life.)
The book is excellent on many levels, difficult on others. As a work of art, it is unusually dense and all-encompassing. Almost against his will, the reader is drawn in as the main character's fate unfolds, brought about what one could call his willful passivity. The plot and character "development" are of fascinating, unparalleled strangeness. At the same time, assessing the novel's intended meaning is a perplexing task. The book's "hero" (for he is often referred to as such) seems to be anything but heroic. Rather, he could be seen as a walking advertisement for the perils of the undesirable traits he possesses. His defining character trait is stagnation. All of his second-hand philosophical posturing is merely a lame attempt to justify his disdain for exertion and his cowardly withdrawal from pursuing a purposeful life in the "flatland" below. (However, Mann - as well as many of the reviewers here - apparently really did consider Hans a hero engaged in the act of philosophical self-improvement. Strange.) Lacking a self, the hero's views and even personality traits are lifted from those around him. (Witness his shameless incorporation of Peepercorn's affectations.) The character most vocal in his defense of virtue (Settembrini) is, on the whole, not particularly virtuous himself. The character presented as the most virtuous (Joachim) is neither the happier nor the more prosperous for his virtue. Furthermore, it is often difficult for the reader to discern whether the narrator's praise for a character is intended to be sincere or ironic.
To a non-literati like me, the author's approach to his craft is often suspect. Momentum is often dispersed by questionable digressions; new major characters are introduced up until the end; fifteen pages are often used where three would suffice. Mann seems intent on presenting himself as a renaissance man, one who can write expertly on a plethora of subjects. He is not grandstanding or hotdogging - it's just the way he writes - but it does require patience on the part of the reader who may not particularly care to detour through a discourse on snake venom while trying to advance through the story.
Before embarking on this endeavor, I was hoping to be able to dismiss the standard Objectivist (i.e., cult of Ayn Rand) objection to this work - that the philosophizing contained within exists for its own sake and isn't integrated into the plot or theme of the novel. The criticism seems to be unjustified as the passages in question unfold. The philosophical views expressed are relevant to the theme and to the development of the characters who express them. After a while, however, I had to concede the point. On and on these dialogues go, completely dissociated from the rest of the book, requiring mental brute force just to plow through them. Score one for the Objectivists.
To the serious student of literature, I can recommend this book unequivocally. To the average reader - even a fairly serious one - the cost-benefit ratio here does not justify the considerable investment of time required to get through Mann's masterpiece. I mean, to me Crime and Punishment is a real page-turner, but I had to force my way through lengthy passages of The Magic Mountain on numerous occasions. I rate it three stars, rather than four or five, for (what I perceive to be) its literary limitations. However as I mentioned earlier, this rating is fairly arbitrary. To the right reader, this could be one of the greatest books ever written.