I'm using RS Components free "Design Spark Mechanical" which is a slightly hobbled version of a full price package called "Space Claim". Apart from the bit they deliberately hobbled "Mirroring, or vertical and Horizontal Flip" using it came quite easily, having taken Engineering Drawing at O-Level must have helped. Also the fact that I had to learn, to at least a basic level, multiple pixel and vector graphic editing, presentation and DTP software packages probably gives me a head start when faced with something new. I'm using it as a benchmark to look at other software, the only freebie that I think comes near it is "Sketch Up Make" but I still prefer DSM's conventional elevation representations and scaling facilities. I'm just on the point of mastering (a few hours more practice needed) the operation of drawing the 3D model of an object, in the current case a double ended motor with two worms meshed to gears, then punching it into the centre of a solid block to create the core of a motor bogie. I did have to try building it from standard shapes then comparing the images to convince myself that I could print what I had drawn.
The printer comes with it's own software to analyse the CAD drawings, carry out the "slicing operation" and output the printing instructions in a widely used form. Just for curiosities sake I did feed it a PDF of a simplified Escher circular waterfall, it very sensibly gave up after a few minutes, so it has sufficient AI to recognise an impossibility when given one.