G.B. Shaw liked Little Dorrit most of all Dickens' novels. It's not hard to see why—though often overshadowed by the accessibility of A Tale of Two Cities or the first-person appeal of Great Expectations, it is essentially the bridge between Dickens' initial style and his more mature work. Understated and realistic in a way the earlier books were not, it is less a hobby-horse for the author's social concerns and more an exploration of the human spirit and the individual pursuit of dignity and legacy.
Old Mr. Dorrit is funny, there's no doubt about it. His affectations of nobility whilst in debtors' prison are comicly absurd, as is his consummate dismissal of the other prisoners. But he is also deeply tragic. When he is still assuming airs in the midst of a debilitating fever, we are more prone to cry than laugh. His treatment of everyone, including (perhaps especially) his own family, is not comedic inasmuch as it reveals his capacity—no, our collective capacity—for selfishness, even when it is destructive. Dickens' humor is still alive and well here, of course, but there are more tears, more reflection and quiet moments of tenderness and sadness.
Amy Dorrit (the little one) is a paragon of virtue. But while many readers (who I'm pretty sure can only call themselves such in the most cursory sense) criticize Dickens for the over-sentimentality of his female leads, claiming they are "perfect" and therefore unrealistic, her virtue is not seamless. Her demuring nature may be beneficial in certain circumstances, but Dickens suggests throughout that her inability to stand up to her overbearing father is not such a virtue as Victorian society would have it. That Arthur Clennam is there to rescue her is fortuitous—other young women were not so fortunate.
Then there's the prose itself. Dickens was a stylist without equal—only Shakespeare among English writers achieved Dickens' stylistic elasticity, embracing every aspect of life from the most sacred to the most profane through sheer brilliance of word-craftsmanship. Little Dorrit reads like an extended poem, an ode to beauty, an ode to life itself. Whether you love Dickens or are wary of his lengthy novels, this one should be near the top of your reading list.