I think they are all right. I have seen them done well and I have seen some really bad ones. I think it all comes down to two things. The context and the actual speed. I tend to almost always prefer to have something on screen to be running from as opposed to just running from the borders of the screen itself. Give me lava, or a big wave, maybe a rock or something. As for the speed, I don't ever want to be running up against a wall at the edge of the screen. It is cool to force me to keep my momentum but don't be so slow that you can't keep up with me while doing it.