For instance for mechanical engineering; from what I've seen, Blender is quite flexible and powerful so maybe it serves well for this also?
It really depends what you are doing exactly.
In general, I would say it's not good as a CAD tool for mechanical engineering.
It's good for modeling good looking things (teaspot, tree, people etc.), but if you want to for example handle strength calculations, you are basically out of luck (unless you do everything manually).
Of course, if you have to sell something, and you need good-looking renderings, then Blender might be good tool, but that's different from designing machines or buildings from engineering point of view.
There exists project BlenderCAD, but I didn't tried it yet. http://sourceforge.net/projects/blendercad/