Being Korean, my family uses them at home all the time, unless a small number of guests are coming (in which case we break out the SILVER chopsticks) and when there are a large number of guests/children (in which case we just use wooden ones). They are not found easily at restaurants, but it seems that the majority of Koreans use them.
As eric+ said before, it is different than using wooden chopsticks. First of all, you cannot leave them in very hot soup for more than a couple of minutes, or else they will get hot and will be hard to pick up. I, having used them all my life, find them easier to use than the wooden variety, but I understand how they might be more difficult. It is harder to use good chopstick form when using them.
They are indeed good for stabbing things. I do it all the time (I'm not sure about how often others do it) with flat, stacked, soft foods.
I'm not sure about why people started using metal chopsticks, but they do have some advantages, including:
- Easy to clean
- Long-lasting
- Easy to mass-produce
As for the "proper usage" issue, a gringo could learn to use chopsticks better than I in a couple of minutes, and get used to it in a week.