The book is very long and at times a bit of a slog. It needs to be because that is part of the point. The main character a completely ordinary man comes up to a sanatorium in the Swiss Alps for a three week visit and ends up staying for seven years.
The book is an allegory of a diseased Europe immediately prior to the outbreak of World War I. The characters come from the major countries of Europe and represent the major philosophical trends and political trends of the time. The book also explores the nature of time. It does this very well, in my opinion and this was my favorite part of the book. In a way not much happens in the book, although it is not slow, but time and events become more muddled.
The author apparently said that it wasn't possible to understand the book, unless you read it twice, something that I am unlikely to do.
Its a well written thoughtful complex book, but it does not grab me in the way, say Dostoevsky does who is perhaps the best comparison. In the end I found it admirable, but lacking that certain indefinable magic that the best books have.