My girl Shadow was a feral that had kittens in our garage a few years ago.
I trapped her and got her spayed, and a friend socialized the babies (5 weeks old) and adopted them out.
We kept Shadow in a spare room for a few days, hoping to keep her as a pet. She hid behind everything, only coming out to eat. One night I startled her, and she was in between hiding places. She crouched and hissed at me, then growled. I gave up, thinking we'd failed and she'd be happier outside, so we released her.
She kept coming around for food, staying at a safe distance. With cold weather coming on, I worried about her surviving the winter, so we put a pet door in our cellar door, leaving it propped open. She came in for food, I sat across the cellar from her just talked to her. Eventually she learned to come up the cellar stairs into the kitchen for feedings. We got to thinking about how short her life would be outdoors (coyotes, disease, busy road out front), and we decided to shut the pet door, keeping her inside.
She became friendly, but wouldn't let us touch her for months. Eventually, we were able to pet her, and she really bonded to me, and to my husband to some extent. He can pet her, but she's a little wary, they can slow blink back and forth. :)
Whoever said patience was very wise, it just takes time. Shadow is extremely timid (not aggressive at all), and she had to figure out I was no threat to her. She can easily go to fear and revert, but mostly she's a happy kitty, purrs a lot of the time, and loves to be groomed. Super happy ending.
However, she will never learn to love music -- especially harmonica. She will leave the room when I play or sing, complaining over her shoulder on the way.
Here's a video I made of her last year: