Shakespeare paid Cervantes (his contemporary) the rare compliment of using Quixote as source material for one of his later plays, Cardenio (unfortunately lost.) The novel has been viewed as an allegory for everything--Christianity, the Romantic cult of the artist, extreme materialism, and the infinite referentiality of texts. Don Quixote is one of the few books that merits casual references with the definite article ("The Quixote"), and additionally is one of the few books to spawn a universally-recognized adjective ("quixotic"). How to even approach a book like Don Quixote, a book that has been, at some time or other, all things to all people? How to evaluate a cultural monolith?
The simplest way, of course, is just to pay attention to the fact that Don Quixote, four hundred years after its initial publication, is still a hell of a read!
Sure, there are rough patches, yet: the mini-novels that interrupt the narrative of the first part for a hundred-odd pages would have been easy targets for some modern publisher's blue pencil, the long essays on arms or piety can ring strangely to reader sensibilities, the descriptions are sometimes a vague mess, and Sancho Panza's brief solo adventures read like the winners of a "Find-The-Best-Tired-Fable" competition and are best forgotten. And yet the basic story, the basic concept holds up: even the irascible Nabokov, in his Lectures on Don Quixote (intended as a six-lecture trashing of the novel), is forced to admit that there might be something to the central character after all.
It's hard to stay mad at Don Quixote: as frustrating as the plot can be at times, some archetypal lure lurks within the world of Cervantes's Spain, some magic that draws us in, much like the world of chivalry that continues to draw Quixote himself through the progressively more painful wringer of situations.
The concept of the novel is simple: Alonso Quijano, landowner from La Mancha, is obsessed with his library of chivalrous books. Driven mad by the inconsistencies of plot, character and philosophy that fill each volume of these seventeenth-century precursors to the fantasy novel, Quijano resolves to restore dignity to the lost profession of knight-errantry, assembles a rudimentary sword, suit of armor, and horse (the eternally-suffering-and-spavined Rocinante), and sets out into Spain in his quest for glory.
In return for this act of hysterical faith, he finds violent innkeepers, malevolent thieves, cynical shepherds, sadistic nobility, and even (due to Avellaneda's false sequel to the book's first volume, one of the most famous pieces of fan-fiction ever written) an inferior (and, in the novel, invisible) Quixote impostor.