FPrime is an almost real-time rendering application. Unlike Viper, which only shows a small, low resolution image lacking in most features, FPrime will render most of what the Lightwave renderer is capable of. However, it can do this in a fraction of the time.
Where it really comes into its own however is for real-time placing of lights, etc. You can get pretty much instant feedback on lighting changes. Try doing that with Viper or F9 !!!
I'd check out the FPrime pages on
www.worley.com - there's some demonstration videos on there that shows it in action.