Let me begin with an explanation of why I prefer unabridged to abridged versions. An abridgement is a version that is shorter, usually heavily edited to streamline the narrative. The problem with abridging is that the parts that are excised are usually the parts that give the book its unique feel. While a plot is important to a story, when we buy a novel we are not buying a story; we are buying the way it is told.
Victor Hugo is the king of digressions, diversions, tangents, and red herrings. He would seem to be a prime target for abridgement. However, once you have done that, you remove the history, the societal context, and much of what makes his novels unique. For example, I once picked up an abridgement of Les Misérables, and discovered that the version started several chapters in. The first part of the novel does not deal with Jean Valjean, but instead with the kindly Bishop of D-- (usually assumed to be Digne). The part the abridgement had skipped dealt entirely with narratives illustrating the Bishop's charitable nature, which is essential for understanding his interactions with Jean Valjean later.
If you have seen the musical, the interesting part is that most of the actual plot of the novel made it in. Hugo's digressions last for chapters and are not often directly related to the plot. (A particularly memorable one is his twenty-odd-chapter explanation of the battle of Waterloo, a part I regularly skip on re-reads but something that might be of interest to the military scholar.) However, it's the little bits that slip out, such as the very fact that referring to Napoloeon as Buonaparte marks you as of a particular fashion, that bring the sense of mid-nineteenth century France to life. One could almost use the novel as a snapshot of the culture of the time, illustrated by the lowest components of society, the 'misérables' of the title.
I do, however, wish that my version of the novel (in two books!) translated all the French. I took Spanish instead, and wish I knew what some of those enticing couplets meant. Still, this is one of my favorite novels of all time, and if you are not the sort to be put off by length, I can highly recommend it.