First and foremost, 'The New Magdalen' has a radically different structure from the other Collins novels I've read. Part of the author's strengths is his use of rich language in describing scenery, characters, and motion. Unfortunately 'The New Magdalen' actually reads like a play. Nearly all of the 'action' takes place in a single drawing room. But 'The New Magdalen' is not a play, and reads instead like a claustrophobic novel.
Beyond this, 'The New Magdalen' tells of a basic story without much in terms of surprise or punch. This is so disappointing when comparing this novel to his wonderful 'The Woman in White'. Instead 'The New Magdalen' is a story of a poor woman who impersonates a rich woman, gets caught, and then too much time is devoted to melodramatic exchanges about what to think and do. No new ground is broken in terms of class prejudices, the hunger for money, etc.
Having said all this, there are some glimmers of the old Wilkie Collins worth savouring. Here and there the dialogue does shine. 'The New Magdalen' could have been a good play. But as a novel it does not rise above average.