This patch adds simple preset menu for unit scale scene property. As illustrated below:
Comparison between old and proposed Units Panel UI:
Issue with current UI is that it's quite obscure comparing to other 3D software:
As you can see other software that supports multiple unit systems provide users with clear and intuitive list. Blender has only float property which does not even indicate itself very well, so users just ignore it and continue to use default unit scale, even if they work in smaller scales.
For example: jewelry and product designers work with millimeters, but default scale for metric system is meters, so in this case all default primitives created with enormous sizes, which continuously frustrate users, because they have to scale them down every time.
And don't think this is a newbie problem, I personally know professionals who work with Blender for more than 5 years, and they had no idea they could work on millimeter scale.
Ideally it would be better to get rid of current float slider and use enum list only, providing users with float slider only if they chose to use custom scale (like in other 3D software), but I guess that will create compatibility issues. So preset menu is the second best choice in this situation.