At about 1,400 pages, the unabridged Les Miserables was the longest, and possibly the most challenging, thing I've read; however, the experience was well worth it. At times, the wretchedness of one situation or another is enough to make one groan aloud in sympathy for a character, but the moments of joy are positively radiant. To comment on the characters: Cosette is heartbreaking, charming, and all-around loveable as a small child, but as she matures, she becomes less interesting and less likeable, and by the time she falls in love, she has become one-dimensional and all-too-perfect; in other words, unbearable. Jean Valjean, however, is a wonderfully developed, very believable, very human character, and compensates thoroughly by becoming increasingly compelling.
As a heads-up, this is Romantic literature and it is extremely verbose; Hugo sometimes spends chapter after chapter discussing something that could be described sufficiently in a paragraph or two. Nonetheless, I do think it's preferable to read the unabridged version; my philosophy concerning the "abridged or unabridged" dilemma is this: if the author likes to take his time, let him. I have not read an abridged version of Les Mis, but when reading abridged editions of other classics, my experience has been you definitely get the sense that chunks of the book are missing- and because you're reading a greatly shortened version, you're always surprised to reach the ending: "So soon?"
In conclusion, this is an excellent book, and I recommend it.