Do these have to be "pretty" because you will show them to customers, or will "utilitarian" suffice?

For plain old utility, if I just need to do something simple, very quickly, is still pencil and paper! I can get something drawn, and be on to other things, is much less time than almost any tool I've ever used.

If you need pretty, then the decision is much more highly based on what you'll be drawing. For drawings of metal, sheet metal, etc... any CAD program will work, but if you've never used 3D cad software, get ready for a pretty steep learning curve. From what you're saying, I doubt you need 3D CAD software. SketchUp is cheap, but a bit buggy if you really need to do things like generate STL files for others. AutoCAD will not likely be something you want to use for any casual use, as it is really pricey. It isn't even thought of as among the best packages out there anymore.

If you're willing to just use a 2D tool, I like Visio, as you can make shapes, and then interconnect those shapes. Doing this takes some effort, but once you have shapes built for your commonly used items, future drawings take much less time, as you just reuse the shapes you've built, and just interconnnect them differently.