I had no idea what I was getting into when I bought this book. But it turned out that I enjoyed it, and although it was more tedious to read towards the end, I would recommend it to anyone who can speed read.
To me this book was written as one incredibly long train of thought. The train itself breaks off into other smaller trains of thought, but it always goes back to the principle story: that of Tomas and Tereza. While Kundera may turn off the reader who doesn't enjoy straight story-telling, he does tell a story here. It's not just a book of random musings and incoherent philosophizing.
That said, the stories of "Unbearable.."'s characters are simple enough. Tomas is a philanderer, torn between his lifestyle and his love for Tereza, who kind of fell into his life by chance. Tereza is his wife, who is tortured by his infidelity but cannot leave him. Other more minor characters include Sabina, a mistress of Tomas, and Franz, another married lover of Sabina.
These four characters are Kundera's chosen examples of the human experience. He reveals their inner desires and motives, and otherwise tells their psychological stories along with their real-life stories. They each have "issues", as does everyone in this world. But it's interesting how their personal philosophies, having been shaped by both their human experience and their intrinsic individuality, are so different from each other's. This in return shapes the experiences they have with each other. Tereza and Tomas lived for so long together, yet they never really thought alike. And because of this, they lived totally separate lives.
That, in full, is my take on the book. Kundera presents many other theories on the human experience, and I found them all interesting, but the one element that I found carried the book through was the variance in the characters' personal (as in mental, emotional, and psychological) life experience. This variance made a whole world of difference, because what is life, outside how we perceive it?
The real-life stories are also interesting, but I think they are meant to be in the background. The main story is mental, it's in their reactions to life, which drives their future actions. I say this because their lives end quite insignificantly, as though they might as well not have lived--a phrase in the book proves this "What happens but once might as well not have happened at all."
And yes, if our lives are perceived this way, we might as well rule them out as insignificant. Our lives can be taken so lightly that is in unbearable--the unbearable lightness of being. But Kundera makes this point in the beginning: his characters are merely that. Characters. He uses them to illustrate his theories on the human experience.
So is this book a negative commentary on life's insignificance? Is Kundera trying to tell us that life means nothing? I doubt that. I think viewed from the outside, our lives might seem like they mean nothing. But to each of us, our life is colored and perceived by what we bring to it: by our history, our philosophy, our dreams. Life is a personal experience, and if it means nothing to everyone else, it at least means something to us, for we are the ones who live it.