The plot of TULOB isn't really the point. Kundera is interested in the inner lives of his characters, and a busy plot would diminish his ability to do this. Anyhow, I found the story of Thomas and Tereza quite involving, and it describes a satisfying arc: everything they have is stripped away by time and cirumstances, until only their love for each other remains - which they realise is the only thing that mattered anyway.
"...but towards the end, it slackened very much. The very ending of the plot frustrated me very much...I never picked up another book by Kundera again."
The ending (with the moth) is subtle, but I thought it was brilliantly done. It hearkens back to a scene earlier in the book.
I didn't feel the book slackened at all, in fact I thought it was brilliantly constructed - the way in Part 6 the action moves to Franz and his ill-fated trip to Cambodia sort of sends the novel up a gear. Then there's the last part with the scene involving Karenin, the dog... I almost felt that everything there was to say about life, was said in those few pages.
And the way we know how the story ends half-way through... All very subtly done - I think you have to be quite attentive to appreciate just how well-constructed the novel is.
TULOB is easily Kundera's best novel, in my opinion, but many of the others are interesting. I enjoyed "Immortality". TBOLAF was OK but pretentious in places.
The later novels have been disappointing.
The non-fiction works "The Art of the Novel" and "Testaments Betrayed" are, in my opinion, the next best things Kundera wrote to TULOB - packed with brilliant insights.
At its best, Kundera's prose is like a lightning bolt: you feel the secrets of existence have been momentarily illuminated for you. His language is so concise and to the point. Sometimes he almost contradicts himself, but as in poetry, he's kind of more interested in what seems to be true than in saying, "This is the case".
Kundera is one of those writers you either love or don't "get" at all.