I would say that for me Bryce 3 to 4 was a worthwhile upgrade, and the price wasn't excessive (can't remember the exact figure, about $100). I'm not sure what features they can add to B5 that would have me making another upgrade. A better modelling capacity would be very nice, but somehow I suspect it just wouldn't work with the way Bryce is structured (consider the "fake Booleans"). Vegetation systems don't interest me that much; I'm happy to add the odd tree by hand. Slower renders I DON'T need, even if they do bring in radiosity.