15fps is plenty fluid, There is a formula for what type of scene needs how many fps. 15fps is perceived real-time and 30fps is true real-time. Many people use lower then 15fps, It all depends on the scene and what your trying to capture. I run all my cameras @15fps because that's more then enough for me. Have a look at the links below, They should give you a better understanding.
Comparison of Frames Per Second for IP Video Surveillance
Average frame rate used for recording
http://ipvm.com/updates/1100
Look at post# 4 for a good explanation:
http://www.cctvforum.com/viewtopic.php?f=19&t=37141
To sum it all up--industry guidelines for deciding on frame-rates for specific settings:
Casino tables - 30 fps
Cash counting settings (bank tellers) - 12 to 15 fps
Vehicular traffic (not at traffic stops and parking lots) - 15 fps
Pedestrian traffic - 5 fps
Parking lots, traffic cameras, overview scenes - 1 to 3 fps
Low-activity settings - 1 fps
After looking at my links I provided it should give you a good understanding and hopefully answer your questions about fps.