This video shows how your footage will look when slowed down to 3/4 speed, 1/2 speed, 1/4 speed, etc. This was filmed with a GoPro Hero3 at 720p 60 frames per second. You can see it starts to get choppy at about 15-20 frames per second, so if you want slow down your footage even more and have it still be 'smooth' (not choppy), you'll have to film at 120 frames per second or more.

For reference, the GoPro Hero2 specs:

1080p 30fps
720p 60fps
WVGA (480p) 120 fps

GoPro Hero3 Black
1080p 60 fps
720p 120 fps
WVGA (480p) 240 fps


I hope this helps someone out there!