FIVe3D is unlike other 3D engines ( such as Papervision or Alternitva3D ) in it's approach. We're not talking shape primatives here, we're talking extending the basic flash elements ( Sprites, Text, MovieClips ) to include Z-properties. Looking at the demos, it's not what you'd expect from 'Flash 3D', because we're expecting another Papervision clone. But no, the approach that Mathieu has taken when developing this is much different - and actually makes much more sense for a lot of situations.
Plus, it's fun. I'm a sucker for fun demos.