I stir fried it once. But I didn't like the oil to vegetable ratio, so I mostly use it in soup. I slice it in fairly narrow strips and put the thicker, white parts of the bok choy in the soup about three minutes before done cooking, and the leafier parts of the bok choy in about a minute later.
I've thought about cooking it with egg, the way I sometimes do with cabbage or spinach, but I can't remember if I've ever made it that far with bok choy. I usually pick up several instant soup packages when I'm in picking up bok choy, and by the time I'm done eating those soups the bok choy is gone.