Melmoth the Wanderer is the story of Melmoth, who sells his soul for immortality, becoming a kind of Wandering-Jew figure. It tells snippets of his story throughout the ages (and needless to say, it all goes horribly downhill quite quickly). It is very much a novel of its time, revelling in the gothic taste for the terrible, the dark side of "sublime" and general death 'n' horror. Now, I actually quite like this genre, and as a specimen of its type it's not at all bad. It is well-written, it covers a whole variety of times and places and brings the full gamut of the terrible into play. In fact, as an essay piece or a reader for a literary course it's probably quite useful.
However, as your common or garden reading-material it's possibly the most depressing thing I've ever read. It takes the floridity of the genre to its full extent - and then out the other side. It involves very few characters that are actually appealing, and probably something dire happens to them anyway. Because it is a series of vignettes, it comes across as being quite choppy, with little in the way of continuity possible. The writing is so powerful that it goes too far and overdoes it, and if you're reading for pleasure (as I was)you may find it a heavy slog.
One of the other reviewers here mentioned "The Monk" and actually I found that a much better balance of dark and lighter writing. This is important as in my opinion, without the lighter bits, the horror loses definition and just becomes a bit...well...overdone.
My view is that if you're dithering between the two, go for the "The Monk". (Feel free to call me a Philistine though!)
JAC.