I actually liked Fanny.
I didn't think that we were supposed to, but I rather liked her. Fanny grew on me a lot by the end of the series. She's a nice contrast to Amy, who is all sweetness and light (which is great for a literary character but can get exasperating in real life). Fanny makes no bones about what she wants out of life; she doesn't pretend to be something she's not, and in a way I felt like her and Mr. Sparkler (great name, Dickens!) deserve each other.
The scene between her and Amy in their nightgowns, particularly, showed us that Fanny has another side. Not to mention that without that silly lipstick on, the actress who played her is as cute as the proverbial button.
"Hey, that's right... We're supposed to sing about pirate-y things!" - Mr. Lunt