99% of the time I'm a staunch purist. A classic book should be portrayed on film as close to the original as possible, leaving room for the necessary conversions of text to screen. That being said, there are exceptions to every rule, and this movie is one of them.
I had not read the book prior to watching this movie, but had read IMDb reviews that it was far from accurate, so I was skeptical going into it. The movie, in fact, was outstanding! I was riveted, drawn into the story, and anxious to find out what happened next. It was fascinating and intriguing. I think the best comparison I can make with it is a Dickens story set about 150 years earlier than his books. It's dark and gritty, highlighting the lowlifes of society and the shortcomings of the society that contributes to make these lowlifes. Yet there is considerable irony and a bit of humor to counteract the darkness. Love, life, death, joy, grief, sickness, deprivation, aspiration.... It is in short a microcosm of life as it is today, but through the window of days past. One feels the emotions that the characters are experiencing, because they are feelings we've already experienced ourselves. However, although this movie is frequently tragic, it is not a tragedy. I could not recommend it so highly if it was, because I don't like to walk away from a movie feeling depressed.
I liked the movie so much that I began reading the book that very night, and I finished it 8 days later. Where did the book differ from the movie? It would be easier to state where they resembled each other! I would have been hard-pressed to see any similarity between the two had they been published under different titles. For one thing, the book covers the title character's entire life up past the age of 60, whereas the movie only takes her up to maybe 30, and what goes on in that time frame is widely different from what goes on in the book.
In the movie makers' defense, they do have in the opening credits the following caveat: "Based on a character in a novel by Daniel Defoe". Okay, so it's based on the CHARACTER of Moll Flanders, not on the story itself. That's a legitimate, though tiny-print, concept. However, even the CHARACTER of Moll Flanders in the movie is quite different from the book. One big digression (out of many) is that movie-Moll has strong paternal, motherly characteristics, whereas book-Moll has essentially none. Secondly, Moll's name isn't even really Moll in the book; it never tells us her real name, and "Moll Flanders" is merely one of her many aliases, and one that she doesn't pick up until her 50s.
I can't possibly go into all the digressions. It would be boring and overextend the 1000 words limit. Suffice it to say, there is scarcely a shadow of similarity between the one and the other. If you have read the book, disregard the title and watch this as its own entity. If you haven't read the book, no need to worry about seeing any spoilers that might ruin a future reading. There is almost no overlap.
However, in spite of this "sin" of gross inaccuracy, I LOVED the movie, and I thought the book was only mediocre. The movie had a great plot development; the book has almost no plot. It was first published in 1722 and, like much early fiction, is mostly just a chain of events. We are told the many escapades of Moll Flanders, but there is no real story arc. (I'm not saying don't read it, just know what you're getting into; it's interesting, but not terribly fulfilling as a novel, in the modern sense of the genre.)
I was impressed with Robin Wright's performance (as Moll). I had thought little of her acting in "Princess Bride", and almost didn't watch the film because she had the title role; but she did a much better job in this film, and showed a much fuller range of acting ability and emotion. The other actors also filled their roles superbly. And the settings and costumes were magnificently done.
One thing that may affect your opinion of this movie is the content. By all standards I've ever watched, this ought to be R-rated. It's not extremely explicit (hence it is not in fact R), but it is considerably so for a PG-13, and I would be cautious showing this to young viewers. Not just for sexuality, but also for some rather graphic scenes (including blood) and overall mature themes.
However, for a mature viewer I think this was a fantastic movie with a great storyline. It's very thought-provoking, and the impression of it still lingers with me a couple weeks later. The writers used Defoe's book as a springboard to better ideas and a more cohesive and rewarding story. It would have been more accurate to have said that it was "inspired by" Defoe's book, rather than "based on", but it is one of those rare cases where the movie is in fact better than the book.