I had never had it (or heard of it) until I moved to Texas. It's called "fajita", and since I started cooking fajita over 20 years ago, I've always served it in a flour tortilla, which is the usual manner (down here, anyway).
Many of the "fajitas" you order in restaurants are not made with skirt steak, and there is certainly no such thing as a "chicken fajita".
It's funny, the first few times I cooked it, I cut it WITH the grain since that looked like the right thing to do. Ah, the foolishness of youth. After wrestling the first few to the ground trying to take bites, I realized my error.
I've tried many, many things to marinate, and I don't remember any one that I didn't like. However, I many times will just use Wish Bone Italian salad dressing. I've had people rave about how good they are, and at times I've acted as if it was some secret marinade passed down in my family.
I like it best grilled over charcoal.
I always make pico de gallo (chopped tomatoes, onions, and jalapenos mixed together; and NO, I do NOT put that nasty cilantro in mine, although most people down here do). I shred some cheese; Mexican white cheese works well, but I've also used cheddar, put out some sour cream and whatever else anyone wants (some like lettuce, salsa, refried beans, etc.), and serve with flour tortillas heated on the grill.
I prefer mine with just pico and cheese, but I also like a little sour cream on there at times.
It's been a few weeks since I've made some. Now I know what we're having for dinner Saturday!