I've been avoiding posting on this forum of late because so many topics turn into competitions between favorites. However, I know and respect David and his work (Hello David!) so I'll chime in for whatever it's worth. I've been drafting with various CAD programs for 25 +- years on both Mac (MacDraft, MiniCad/VectorWorks,PowerCadd, Form•Z, & Bozai) and PC (AutoCad).
The first thing I notice in the replies is how quickly (pretty much immediately) the conversation turned to CAM & CNC, when the original query was for a CAD suggestion.
As with choosing any tool, what do you want to do with it? If your goal is 2-d working drawings, there are many drafting programs that are fast and easy to use. I use PowerCadd (Mac) which is a very fast and elegant program. I draw guitars in full and as parts, print out full size on my 24" wide plotter and presto I've got plans. Sometimes I'll take the full size print and spray adhesive it to 1/4" ply or mdf, cut it out and I've got a hard pattern I can make parts from. I can also take 2-d vector based files, e-mail them to my laser cutting guy and he can cut out the 1/4" ply to tolerences in the 1/10k" tolerance range.
Now, if you're planning to dive into the world of CNC, that's a different animal. The sub-questions this creates are will you be subcontracting out the CNC work (in which case you don't need to generate the CAM file, your sub will) or do you plan to buy a CNC machine and jump into that deep and dark hole yourself?
The modeling programs already listed all have their strengths and weaknesses. I personally use Form•Z (runs on both platforms), which can output any number of file formats that CAM software can adapt to a CAM file. I can also generate quicktime animations that are an invaluable presentation tool to my customers.
http://www.charlesfreeborn.com/MissionTableMovie.htmlSpeaking from experience, the most expensive part of any 3-d modeling package is the time it takes to get really good at it. If you are a complete neophyte at drawing on a computer I suggest you start very simply with something like Google Sketch up (free). Then really evaluate what you want to do, and go from there. Form•Z has a streamlined version called Bonzai that is about $500 and will have you modeling in very short order, with plenty of tools and capabilities for drawing, but no CAM output. It can however export files to CAM software, which puts us right back to the CAD/CAM/CNC decision. I say don't buy a Ferrari if a bicycle is all you need to get you there.
-C