I've wanted to read this book for a long time. Specifically, since I saw the movie as a teenager. A boyfriend took me to the movie at the time, presumably for it's titillating qualities -- indeed, I'd call the movie 'soft porn'. Anyway, I didn't really understand the characters in the movie. I mean, I was a teenager and all, but even then I couldn't understand their motivations, didn't get what they were about.
I'm so glad I read the book. I'd like to see the movie again now, but from what I remember, the movie is very different than the book. The book goes into more detail. Tereza, who in the movie is a weak, waif like dependent wife, is really an abandoned child looking for someone with strength to take care of her. But also, she understands a lot more than it appears. These things are able to be fleshed out in a book format, but not so much in a movie. So, in the book, you get more of a character study of each character and why they are behaving in such manners.
Having said that, I didn't really like the book all that much. The characters seem unconnected. There isn't really a storyline. You get inside one person's head in one chapter, and another's the next. But it isn't in the context of an ongoing story. In addition, at times Kundera completely steps out of the story altogether and starts talking to the reader about his philosophical ideas which are barely connected to whatever part of the story he has been telling. I really don't like it when authors do this. Tell a story, or write a nonfiction book about your philosophical ideas...but don't try to do both at once. Later in the book, he branches off for entire chapters about his views on religion and politics. By the end, I was skimming over entire pages just to get to the end.
So the final verdict is that I'm glad I read it because of my personal history with the film....but you might not be so glad if you read it. :)