Nancy is an excellent character to analyse in this wonderful book of the Victorian underworld. I definitely think you are right to admire her, as your question states, and one of the principal reasons that you can use to justify your admiration is the way in which she is unique in the novel. The rest of the characters are marked by either being "good" or "bad" with no in-between states. Nancy, however, is a character that shows that in spite of the life she has had and the way that she has been corrupted by Bill Sikes and Fagin, she is still capable of nobility of thought, compassion and kindness.
This is most obviously shown in the way that she acts against Sikes and Fagin to try and save Oliver, and regrets her action in kidnapping him back into the world of crime in which she herself is mired. Note how she is described to us at the beginning of Chapter 40, when she meets with Miss Maylie:
The girl's life had been squandered in the streets and among the most noisome of the stews and dens of London, but there was something of the woman's original nature left in her still; and when she heard a light step approaching the door opposite to that by which she had entered and thought of the wide contrast which the small room would in another moment contain, she felt burdened with the sense of her own deep shame and shrunk as though she could scarecely bear the presence of her with whom she had sought this interview.
This, then, is why we admire Nancy: in spite of her corrupted upbringing and childhood, she still retains something of her "original nature." She feels shame at her state, and yet in spite of this, she acts against those who have power over her, even when she thinks it is likely she will be killed, to save Oliver from the same fate that has corrupted her.