I can see that Blender would be great for creating more "organic" shapre and structures, but when it comes to creating more structured things like enclosures for electrobic circuits, how does Blender compare to something like DesignSpark Mechanical?
I'm not personally familiar with xyz_cad_package, but I would say that if you're already familiar with xyz_cad_package, I would encourage you to continue to use xyz_cad_package if you feel it adequately enables you to make what you like.
That said, in my experience, Blender works quite well for non-organic modeling. http://www.thingiverse.com/thing:83054 is one such example. As for enclosures and what not, the closest example I have to link to is this: http://hipsterpunk.com/u/aeva/m/keyboard-repair-printed-bracket/ and http://hipsterpunk.com/u/aeva/m/keyboard-repair-bracket-test/
I did address this a bit in the article, but here's the relevant section:
Drafting copies of parts within Blender is fairly straightforward. It can be set up to use real-world units for all its functions, making it easy to punch in the dimensions and coordinates of an object's features. Many times, I've used nothing more than calipers, Blender, and a keen eye to create copies of objects, such as enclosures to parts, brackets, and interfaces, or to allow specific components to fit snuggly within a printed object.
While reading this (and the comments) I began to wonder: Has anyone yet invented a 3D scanner so you can copy an existing design? Or, for instance, scan in and remake a part that is broken? I can recall some things that got broken that were impossible to mend - you could stick them together but they'd fail again at the repair point. If you could stick them together so they were the proper shape, scan them and reprint them, that would be a trick worth doing.
3D scanners do exist, though they're rather primitive still. For replacement parts, I've found that usually you can quickly draft a compatible copy of something and print it out. Depends on what broken. If durability is required, you can test the replacement part in plastic first, and then use an online 3D printing service to get it printed in metal.
@David: Has anyone yet invented a 3D scanner so you can copy an existing design?
These have been availavle for a while, but they used to be mega-expensive. More recently, quite a few affordable ones have appeared on the scene (or are starting up as KickStarter projects) -- check out the MakerBot Digitizer, for example.
@Dave 3D Systems has a relatively cheap hand held 3D scanner (~$400) but there are solutions that use a cheap laser and an off the shelf camera. I don't know how good the results are as everyone says their's is great :-)