There are a lot of bugs between the 5.5 and 6 updates that have yet to be fixed. Most notable is animations and material animations are broken. If you try to bolean terrains the result comes out at a different place than you originally positioned it. If you wanted to make a 3d oakleaf by using it's outline and a rolling hills type texture you wind up with a mesh that has long drawn out triangle polys along the edges. 

I don't see much chance of all the problems being fixed without a version 7 upgrade and pumping more money into bryce development. I thought that was what version 6 was for but I guess I was wrong.

On the other hand for 5 times the price you can get Rhino that will model anything but has a basic renderer. For 50 times as much you can get maya or max and have full rendering with effects plus particles, physics, and lots of nifty commands like cage editing. You can get paint effects that lay down random trees and plants that respond to weather and export, the list could go on all day.

For $300-400 like Carrera you could finally get an unbuggy version of Bryce. For about $1000 you could get a version that has most or all of the wishlist items people post about. What are you wiling to spend?