when I had to work with 3D mechanical objects I chose FreeCAD for the following reasons:
1) a very strong and reliable operability for STEP import and STEP export
2) importing and exporting STEP models work preserving face colors even in case of fused objects
3) a strong and reliable scriptable interface
4) a fully GUI programmable interface
5) option to export VRML compatible to kicad
6) Win, Linux, OSX version
7) a very useful and helpful forum community
8) open source project as kicad
I looked also to BRL-CAD but when I tested it, importing STEP models were quite limited/partial supported
currently the tool step-g has partial support, and the toolkit https://github.com/stepcode/stepcode is branched into the src/other source tree.
I don't know if they have improved it but I re-checked quite recently with the same feeling...
FreeCAD learning curve is quite long, as in fact is for many MCAD sw, but FC programmers are developing new features and Workbenches to improve user experience
I recently added the ability to preserve colors to the Assembly2 WB, and that has been merged, so from now assembly parts in FC is even simpler and colorful
and moreover there is kicad StepUp that is developed under FreeCAD, which can help you in ECAD MCAD collaboration, exporting directly your board and parts to STEP
and a lot of mechanical library parts here
EDIT: I forgot to mention DesignSpark Mechanical, which is free but not open source... its usability is quite easy, but it saves only in proprietary format, unless you buy an add-on license. It does import STEP with colors but doesn't export to STEP (free version), so it is not suitable for a bi-directional use...
OnShape is free too (only on line tool), but it doesn't manage colors in STEP files, if they are fused or if the colors are on faces, so you will lose some aesthetic but useful aspects using FreeCAD for KiCad 3D MCAD libraries or on line library models ...