I FaceTime with family during the months when I'm away from home for college, and I loveee it. Last school year I relied on Video chatting on iChat, but it's definitely not as personal as FaceTiming, and it's less convenient because you both have to sit by the computer during the duration of your video conference, and you can't really do much else. But with FaceTime on the iPhone, I can video chat with my parents virtually wherever I want, and my parents don't have to sit by the computer--they could be in the kitchen, outside, or cooking dinner.
The iPad is a similar experience, but I personally prefer using the iPhone to FaceTime, because it's easier to move around and prop against things because of it's small size. Also, because I FaceTime with my family daily or every other day, we sometimes we just like to have the video conference chat open while I'm doing something else, and I like it because I can see them daily and feel like im somewhat in the same room as them. But I sometimes like to multitask on my iPad as well, so that's why I'd choose to FaceTime on the iPhone instead. Though they're both great.