How hard is it to learn to use the program? I've dabbled with similar programs and found them hard to use.
Sketchup takes practice to get it to do what you want, but it's workable, and it's free from Google. They also have a fairly large components library (the components are free to download as well).
I'm considering modelling a few of my ideas for Tequilapoli's weapons systems in Sketchup before I draw them.