XFrog is a very good program for all sorts of "organic" modeling and "abstract" sci-fi type structures. Personally I much prefer Onyxtree for tree modeling, it's much more intuitive to use than XFrog. But Onyxtree has its limitations too.
For Mari-Anne: If you only want to make trees and the likes, the Onyx suite may be the best choice. It's more expensive than XFrog (XFrog US$ 399.00, OnyxGarden suite US$ 495.00) , but you get a lot of models with each of the programs (broadleaf, conifer, palm etc.) Most XFrog users end up having to buy the different packages (e.g. Tropical, Desert, Flowers) because making your own models is difficult (judging from my own experience and a lot of other opinions on the web) And each XFrog package is US$ 199, so you can quickly top the price of the Onyx Garden suite.
Link: http://www.onyxtree.com

I'll follow up by making a demo render with an XFrog tree and a "copycat" Onyx one.

Lars "bigbraader"