Yes, I can think of plenty of novels that have unlikeable main characters that I enjoy very much. I just looked at my personal Top Twelve, and few of them have real "heroes." In fact most of them feature unlikeable characters, e.g., Rabbit Angstrom, Frédéric Moreau, Jake Barnes, the entire "cast" of The Good Soldier, and so on. Not exactly sympathetic. Indeed, I don't appear to be the sort of reader who insists on being able to "identify" with a character. I don't think I identify with characters much, even when they do resemble me. Perhaps it's that Tomas continues his pointless womanizing, even while knowing how much it hurts Tereza. Obviously, he should never have married her. Or he should have broken with her if he found that he could not stop compulsively sleeping with every woman he meets. In any event, I just found him annoying and distasteful in a way that other fundamentally unsympathetic characters do not strike me. It's hard to defend. I feel pity for Rabbit Angstrom, despite his being pretty much a jerk. I'm charmed by Tom Jones, even though he, too, is a womanizer. Heck, I even enjoyed reading the chapters that enter Crosbie's mind in The Small House at Alinngton. But Tomas? Well, he gained a little of my sympathy back in the last chapter with his warmth and kindness toward Karenin. I just wish that he could have shown this side of himself toward another human being. (P.S. I need to read The Return of Casanova. I like Schnitzler very much. I have acted in La Ronde.)
Back in the 1990's, in the wake of the Communist regimes' demise in Eastern Europe, we had a chance to discover the literatures of a number of very interesting countries. Milan Kundera, long exiled in France for his opposition to the Soviet puppets, became a cultural icon, an immensely popular writer, and for good reasons. His surprising blend of novel-essays was refreshing, and for some reason, I didn't read TULOB back then, but "The Immortality" which I liked even better than this one. I saw the movie, though, and liked it a lot as well.
However, I'm not sure that his books are ageing so gracefully. The novel is certainly interesting, analyzing in depth the phenomenon of infidelity and the fact that some persons are easily able to distinguish between sexual desire and love (Thomas, Sabina), while others find it extremely difficult (Theresa). It is a mistery what determines this fact. The other deep discussion, of course, is the basic existential dilemma: to have to choose between mutually exclusive options in a unique, irrepeatable, life. Oh, and the "lightness" or "heavinness" of being. I totally agree with you, Sterling, in disagreeing with Kundera and Nietzsche (whom I have just started to read) on this issue, although I had it in my mind as a diffuse feeling, before you put it into precise words: it is exactly the irrepeatability (does the word exist?) of human life which gives it its heavinness, its life-or-death substantiality: you have just one chance, that's it. Otherwise, if one were to have multiple lives, Life would become lightweight, frivolous, almost irrelevant: what's the problem with dying young while trying to skydive off the Everest summitt, if a few moments later you would wake up again, only to become a degenerate, hedonistic rock star who could comfortably die at 27, then to be born again and try becoming a Nobel Prize in something? Sounds like fun, but it's a pipe dream and would probably becoming boring after a few iterations.
In fact, Kundera implicitly acknowledges this when he says that the lack of life's recurrence is in itself a stimulus for an attentive, reflective life: to be blind to life's coincidences in everyday life is to lose the dimension of Beauty that lies hidden in it, if only we open our eyes and minds to detect it.
The characters DO have a problem in not being likable, not in the sense of "nice" or "endearing" (many of the most literarily liekable characters are despicable, perverse, twisted beings), but in that they take their obsessions to sick extremes, without grace: Thomas's womanizing has nothing of the adventure, sexinness, transgression of moral codes, and sheer fun of, say, Casanova's or other XVIII Century French transgressors, but feels like desperate, sordid attempts at finding something that isn't there because it is not in the seducer's heart. They sound lamentable, rather than enviable. And Theresa's pathetism reaches a nauseating point, the point when fidelity stops being admirable and becomes exasperating (remember Lily Dale?).
(An aside: Arthur Schnitzler's marvelous novella, "The Return of Casanova", portrays the great seducer's decadence and last dance, in grand style, but no less pathetic).
Franz's journey is interesting because it depicts the uneasiness of the sixties with the liberal, capitalistic order of the post-WWII world, especially acute in France. The irony here, to Kundera's merit, is that the whole novel is permeated by Soviet oppression, the permanent and irresoluble tension between freedom and security, pulling us humans all the time in dfiferent directions and forcing us to make painful choices (AH, if only one could freely love and be loved by all women one has ever desired!!), whose consequences we simply will never know, but for one.
TULOB is then, not a frivolous or irrelevant work, but also not a great novel: perhaps it fails just to show the essential failure of life.