Of all the books I've reviewed on here, there are none of them as sweeping as Don Quixote. I began Book I back in June of 2006, and started Book II in January, so it's also been one of the more drawn out reading experiences of my life. That said, it's also been one of the most satisfying.
Summarizing Don Quixote is quite difficult, because describing the basic framework of the story doesn't even come close to doing it justice. Often described as the first modern novel, it manages to touch upon literary devices and genres that have only rose to prominence in the last 50 years, not bad for a book completed shortly after the King James Bible. Between the poetry, the interstitial novels, the occasional breaking of the fifth wall (and the indirect insertion of Cervantes himself into Quixote's word), Don Quixote as a literary powerhouse can't be overstated. But, is it any good?
I found Don Quixote to be an enjoyable read. At some points, particularly in the largely satirical first book, it read just as quickly and easily as any modern novel. In the second book, written not only to complete the story but also to make a statement about deceit, human cruelty, and plagiarism, the novel takes a darker turn, and the jokes played on Quixote and his squire Sancho Panza become so overblown that they inspire sympathy for the once-comic characters. As a result of the weightier second book, it's a bit more difficult to get through, particularly if you tackle it right after the first. I found that once I took some time off and approached book two on its own terms, it was well worth the read.
One last note: if you're considering reading Quixote, I recommend this translation (Edith Grossman). I had started several others previously, and found them amazingly dull.