tl:dr If you are in the outer boroughs of NYC and a car slows down next to you while you are walking, or stops while you are hanging out with friends, you should probably hide your valuables.
Once successfully, a few more times unsuccessfully. It can't be helped when you live next to a not-so-good area in a big city.
The first and only time they actually got anything from me I was maybe 10 or 11. I was waiting on my bike for my friend who ran into Home Depot to get batteries for his Gameboy, and some big guy pushed me off the bike and took it from me, I grabbed on to the wheel to try and stop him from being able to ride and he kicked me a few times before riding off. I was pretty pissed, since it was the first bike I had owned that was bought for me, not just a hand-me-down from one of my siblings or cousins.
I once had a group of teens (I was 17 at the time, pretty sure they went to the same school as me) pull out of a car and try to rob me for my original iPod. I was pretty fit back then, and only a block away from my destination, so I just ran. It caught them by surprise so I got away without much issue. I was pumped afterwards that I got away, it was quite the rush.
Another time I was escorting a drunk friend home from a keg party (I was maybe 16, and I didn't like the idea of him walking home through a really bad area alone at 4am while wasted) when a mini-van drove onto the sidewalk to try and run us down. I pulled him out of the way, then the mini-van stopped and a bunch of guys got out and demanded that we hand over our wallets. I was my usual charming self, which lead to them being extremely pissed off when I started tossing insults to distract them. When one of them came over to me, the car I saw coming had finally pulled up and the driver started honking his horn, which caused all of them to run back into their mini-van and leave. That guy who pulled up and honked probably saved me from a hospital stay. Afterwards I sort of rushed my drunk friend home before they could circle back. I took a bunch of random turns so they couldn't find us easily if they wanted to. My friend swore he'd never walk with me at night again, and he was a man of his word. It was a hell of an adrenaline rush.
A few years back I gave a homeless guy 2$, he pulled a knife and demanded that I give him another single dollar or he'd put me in the hospital, and he'd feel really bad about doing it. I talked him out of it, and convinced him that I couldn't afford to give him any more, so he left me alone.
On a few occasions I've had people try to knock me off my bike, or stop me from riding, to try and steal it. It happens often enough that I generally avoid taking back streets, and try to ride down streets with a lot of stores.
And a bonus one involving my brother. My brother and a group of his friends once got robbed at gunpoint while sitting on our front lawn. They were high and just staring at the sky, some guys pulled up in a car, got out, pulled their guns, and demanded that they hand over their valuables. My brother lost his cell phone and his wallet. They stopped hanging out in front of our house for years after that.
I've been luckier than a lot of my friends and family. I can be really charismatic and talk circles around people to the point that they leave me alone, which has saved me more times than I can count.