I am not all that expert on HD video editing but certainly I am no beginner no more so here’s some advice on how to take good slow-motion HD videos with your Canon 7D:
1. Use the 720P 60fps mode to take video that you want to slow down.
2. Use your favorite video editor to slow it down to 30fps.
When you try to do this with a 1080P 30fps video, you will get a lot of aberations when you slow it down but 60fps to 30fps looks very, very clear.