Little Dorrit is not amongst Dickens' most famous works. I often think with Dickens that the critical reception and popularity of his novels is not a sure fire guide for their relative quality. While, along with everyone, I think that Great Expectations and David Copperfield are the best, I hold the very unpopular opinion that Our Mutual Friend and Barnaby Rudge are better than Oliver Twist and Bleak House, not that anything that Dickens ever wrote was bad.
Little Dorrit is clearly up close to the top of Dickens' oeuvre and it shows him pulling off a number of things that only Dickens could. The book is a comic masterpiece, although the subject matter is quite dark. That's normal for Dickens of course, but it is particularly well done in this work. His characterization is well done. Dickens wrote caricatures, other than the main adult male protagonist, who always appears to be Dickens himself, but here those caricatures are amongst his finest. Dickens' style was always to write a person as if they were entirely composed of one or two exaggerated traits, but he could do this in a way that playfully, or not so playfully lampooned human kind while having this deep insight into human beings. He does that more or less well in different books and here it is close to its best. In particular, the character of Little Dorrit is the best done of Dickens' virtuous females, and actually appears to have some relationship with human kind, which is not usually the case with the heroines of his books.
The rest of the story works incredibly well. The book is full of scathing social critique. I think that here it is a bit better than, say, Bleak House or Hard Times which hammer home their point in a less artful way. The main target is Debtors Prisons, and Dickens is better at slamming things that scarred him than things that didn't and his family was imprisoned in debtor's prison. The sheer delight of Dickens works is here on full display and you can't help admiring sentence after sentence of his prose. The book contains all sorts of typical nineteenth century devices, like sudden inheritances, strange coincidences and orphans. Dickens could use those devices more or less artfully and here they are so well done that you forget to object to them.
A lost masterpiece. It is about on a level with a Tale of Two Cities for me. Highly recommended for anyone who has read some Dickens. For everyone else, they should read Great Expectations right away.