My background is in graphic design so I'm happiest working in Photoshop and Illustrator (or a pencil, or a paintbrush) - I have no CAD experience to speak of. So I'm starting out, like many of you non-engineers, with Sketchup. I've been through a bunch of very helpful tutorials and can now make a simple house, a vase, etc. I love how intuitive it all is, and I'm looking forward to really being able to do some cool things with it.
This week I installed a plug-in that would be useful for many people designing for 3d printing. It's CADspan, it's free, and it helps you detect reversed faces and non-watertightness. It also lets you export an STL file. Did I mention that it's free? It also has a smooth/unsmooth option and various other tools I haven't played around with.
I have yet to actually print any of my own designs, but as soon as I have a free moment I plan to try!