It depends on how you define "stole," but yes, there is a lot of evidence (some coming from Watson's own autobiography) that the Franklin data was appropriated without Franklin's permission (and Franklin had explicitly declined to collaborate with them), and was key to the development of the Watson/Crick model for DNA. This article from Physics Today from some years back summarizes the situation well. I think by most standards of scientific conduct this would be considered inappropriate behavior on behalf of Watson in particular.
No. I don't think they stole it but I don't think the story is entirely fair to Franklin either. Watson/Crick were collaborators with Franklin, but she was often very unfairly treated by both Watson and her supervisor, Maurice Wilkins, who also won the Nobel Prize (which should have been hers if she was alive) with Watson and Crick. They mention in their paper on the DNA helix, which was published back to back with Franklin, that they saw her data, but it gets a little confusing, because they write:
" We have also been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of Dr. M. H. F Wilkins, Dr. R. E. Franklin and their coworkers"
but they earlier state
" We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments"
This is the part that people have been debating since then, and isn't entirely above board and true. They were aware of this because one of the other members of the MRC, Max Perutz, was an advisor to the group where Franklin was and received advance, confidential information about Franklin's data. He shared it with Watson and Crick and they then used it to piece together their model. But I don't think it was theft for a number of reasons, though your interpretation may be different.
There was extensive sharing of ideas and cross-talk between the Cambridge and King's College groups. Rosalind in fact steered Watson and Crick away from an incorrect triple helix which Pauling eventually published.
They ultimately published back to back in the same issue of Nature. Wilkins published Franklin's photo in the same issue with her knowledge, and Wilkins was in constant communication with the Cambridge groups and with Franklin. She fully received credit for her work.
3). She acknowledges the help of the Crick in her paper published in the same issue that shows the data.
4). Franklin herself likely didn't consider it theft and was a close friend of Crick (she didn't like Watson, for good reason) until her death. When she was dying of cancer, she lived with Crick and his wife, so I'm not sure that she would do this if she felt she had her work stolen.
So, its difficult to sort out but I don't think that it was "theft" because there was open communication of most ideas between the two groups and the discovery of the helix was a group endeavor probably not possible alone. Franklin alone would have likely never gotten the structure, since she was a hardcore physics type person who wanted to determine everything directly from the data while Watson and Crick just used models and compared them to the data until they found one that fit. If you break it down, Franklin was an extremely talented crystallographer, Watson had the chemical knowledge of how the bases could pair, and Crick was the theoretical person that could translate between the two. Wilkins made some contributions, but I don't personally think they were as substantial.
Sources: -Rosalind Franklin: The Dark Lady of DNA
What Mad Pursuit: A personal view of scientific discovery
I Wish I'd Made You Angry Earlier: Essays on Science, Scientists, and Humanity
Present at the Flood: How Structural Molecular Biology Came About
The key turn in Watson's and Crick's research into the structure of DNA was a picture by Franklin, shown to them by Wilkins without her knowledge. But in filling their report on DNA's structure, they failed to 'properly' credit Franklin. At the very least, she was snubbed of her proper credit (and in any case, she would've been unable to get a Nobel because she died before they awarded it to Watson and Crick). The accusation that she was robbed, however, comes largely from the fact the she was a woman, who weren't exactly well represented at the time in science.