I have some experience with KiCad but had not used the eagle importer yet.
Some time ago I bumped into:
https://hackaday.io/project/164305-roscom68khttps://github.com/rosco-m68k/rosco_m68kThe author of that project had made it in eagle, but in one of his project blogs he mentioned he was interested in KiCad. I thought it was a good Idea to get a bit of hands-on experience with importing an eagle project in KiCad, so I made him an offer to port the project and he accepted.
I pulled a clone from github, opened it in KiCad and just saved it, and it mostly worked in about 5 to 10 minutes, but it looked quite ugly because KiCad handles labels, buses and bus labels differently. After that I think I spend about an evening (4 to 5 hours) of cleaning it up. Probably more then an hour was for figuring out what that project did in the first place and how the different schematic sheets fit together.
All the cleanup I did was in the schematic. The PCB import was (nearly?) flawless.
As with any decent PCB design suite, KiCad also has a full DRC. This means that if there are any errors during the import, then DRC will flag them as long as both the schematic and the PCB do not have the exact same error (and that would be extremely unlikely).
This does leave some remnants, as KiCad also imports all schematic symbols (including power symbols) from eagle and these do not look very "beautiful" in KiCad. Same for the footprints on the PCB. I easily could have spent much more time on the "cleanup" by replacing imported eagle symbols by native KiCad equivalents, but it was not my project, I reached my goal of verifying the importer works, and I did not want to change too much on someone else's project.
And KiCad is FOSS, which means you can just install it, make a backup copy of one of your eagle projects and then open it in KiCad and see for yourself how good it works.
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About the KiCad version...
KiCad V6 was expected early this year, somewhere around February, and it's being ... delayed a bit. At the moment it's becoming unsure if the Release Candidate for KiCad V6 will be released this year, and the official release of KiCad V6 will probably be a few months later.
The differences between KiCad V5.1.x and KiCad-nightly V5.99 (which will become V6) are quite significant. Starting now with KiCad V5 would seem illogical.
KiCad-nightly V5.99 is quite far developed, but still about 7 issues per day are being fixed. If you're adventurous, or curious, then I'd say give it a try. If your conservative or your bread and butter depends on it, then using it for "serious" work is not such a good idea and you'd probably better wait for KiCad V6.
But if you port an eagle project to KiCad, you still have the eagle project. So there is not much keeping you back to just try how good it works. I also advise to start with something small and not too serious. The Idea is that if you make mistakes out of inexperience, you can just start over without loosing a lot of time. Bigger projects have much more repetitive work, and this makes it more tedious, and is a hindrance to the process of learning KiCad itself. Projects of around 10 to 20 schematic symbols and below 100 pins are probably best to learn with.
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About mechanical CAD...
There is a "KiCadStepUp" workbench for FreeCAD.
In the two and a half minute video below it is used to create a PCB outline from a .STEP file, apply an offset to get a clearance, and then export it to a KiCad project.
https://forum.kicad.info/t/pcb-shape-from-enclosure-step-file/30935/14FreeCAD has a lot of powerful capabilities, but it's not an easy program to learn or use.
It has also made quite some progress in the last 5 or so years.
I've been using it for hobby things for a few years now, and each workbench is a separate struggle to master.