At what point does a book become too sentimental?

I was recently reading some critiques of one of my favorite novels, Les Miserables. Apparently the book was widely disliked among critics for being overly sentimental.

Of course I disagree with that statement, having found the book to be insightful and an interesting read myself. But it got me thinking: when does a book become too sentimental? When do profound words of reflection start to lose their effect on the reader?

And on a similar note, if an author does everything write prose- and plot-wise, can a book still be ruined (for you) through excessive sentimentality?