I recently took a class where we had to use Sketchup. Personally? I hated it. I do admit, there were some useful tools...the Push/Pull tool, and to a certain degree the FollowMe tool, for instance. My problem was that to do really detailed work, there were so many "work-arounds" you had to learn that the general advertising for Sketchup (so easy to use!) seemed like a complete lie. It was NOT user-friendly, and NOT easy to use. At least for me.

I haven't tried to do natural features in Sketchup...it seems to me that Sketchup is most useful for architectural-type work, man-made objects, etc. Trying to do natural objects like trees, landscapes, or living creatures...it just seems like a total waste of time to me. If you are going to do that, you might as well spend your time learning to use a REAL 3D tool like Bryce, Maya, 3D Studio Max, or any of the other professional-grade 3D modeling programs out there (some of which are free, by the way).

That said, if you wanted to throw together some 3D stuff quickly to give you a basis to start with in Photoshop/Gimp...Sketchup is definitely good for that. One professional artist of my acquaintance says he uses Sketchup to throw together a rough prototype of his scene, rotates the view until it's what he wants to draw, and then renders it in 2D. He then plops the 2D into Photoshop and goes to town, safe in the knowledge that the perspective has been figured out for him. So, YMMV.

(But I still hate Sketchup. )