right now, sketch up is one of the greatest things that's come into my life. that might sound gay, but it saved me hours of wrok for one of my architecture class.
Sketch up pro 6 is fucking amazing. However, there arent many differences with the regular and sketch up pro6, but pro lets you play with the shadows for different times of day and year, has a push/pull tool, and it has a bunch of other stuff. look online. I have a student discount thing that lets me use it for a year for $50. Which it is worth it cause in my class I designed something and had to do light studies, and instead of messing with my model in the sun and taking pics every hour outside, I just created my design in sketch up and exported 2d graphics. it saved me so much work
You can make the most bad ass terrain park features with it. I've designed the box for my backyard on it, I've started designing a huge terrain park, ive got like a triple s rail, a trap s box. tons of stuff. It's good shit.