Kinda to add onto what cr0sh is saying, when I built my scrap CNC mill a few years back and I know I certainly underestimated the effort required. You spend so much time with CAD and setting the machine up that it's hard to get things done. First time never comes out right. Unless you're making a lot of parts "production" style it's always easier to make the part manually.
IMO, that's only true if you are already a real machinist and/or your CNC isn't as friendly as it could be. I'm not a machinist and our CNC system is pretty awesome (the touch probe is a big part of the magic). Almost everything made on our CNCs are one-off.
One of the students was ragging on me for using the CNC mill for a drill press (massive overkill), but the touch probe let me get things aligned very accurately and very quickly.
Of course, that touch probe system probably costs more than a "home user" CNC machine.
I suspect there are similar differences in a reprap and a $100k Stratasys, especially if you can throw in Solid Edge or similar professional level CAD/CAM software.
-j