I've used Sketchup for a while and I love it. I haven't used it in the blacksmithing context yet, but its inevitable. I have a background in some of the more heavyweight 3D programs and can say SU is not a full blown 3D modeling program, but it is really fast and simple and I end up using it a lot to plan out things.
Lately I've been planning some woodworking with it. The following site is very helpful for planning multi-piece projects, its woodworking, but I think in principle a lot of it applies to metal work too:
Design. Click. Build.
Also, there are bunches of good videos on youtube, some put out by the Sketchup team themselves, that teach about everything imaginable related to working in SU.