I got my GM5 as a complement to my Nikon D810 (and D700) system, so my comments should be seen in that perspective.
Surprisingly small, surprisingly competent. IQ (especially high ISO) of course not comparable to my FF Nikons, but that was not to be expected.
Body is slippery as a soap, but some gun tape right front and back fixed that. Gun tape is an adhesive rubber tape with a very coarse surface, see guntape.com, recommended.
Electronic shutter was a little bit of a learning experience.
EVF is a nice complement especially in a dark setting with no reading glasses. In bright sunlight the EVF and the screen are both largely inadequate, at least compared to a good Nikon finder.
Battery life is, as is well known, tiny. 2-4 extra batteries seems to be the norm.
With my Nikons I weed out bad shots by protecting the good ones and then using the Delete All function. On the GM5, Protect is quite cumbersome to the point of being unusable, and Delete All is massively slow. So the solution is more and bigger memcards, and do the weeding on computer.
Wifi is a fun little complement but not well thought out, implementation seems to be along the lines of what is easiest to implement rather than what the photographer needs or wants.
The tiny format makes it all but impossible to use defocus to isolate subject from background if you are using a zoom.
Lumix 12-32 f/3.5-5.6 stabilized kit lens:
Impressive little bugger - performs well across the board. There is no focusing ring. Defocus is nice in a neutral way. I use a metal hood rather than a lens cap. Longer reach would be nice but primary differentiator is size so that compromise is acceptable.
Lumix 42.5 f/1.7 stabilized:
Good performer. The relatively large aperture makes it possible to isolate foreground from background.
Good bokeh. Transition from focus to rear defocus goes to blur without softness in between. Front and rear defocus is and evenly lit disc so no artefacts to expect. Wide open there is a slight internal vignetting towards sides and corners but nothing dramatic.
Comes with a plastic bayonet hood.
Lumix 35-100 f/4-5.6 stabilized:
Very good super-compact telezoom, though the small aperture makes it less attractive for isolating subjects. Don't expect this to be a sports action shooter, if subject is 10-20 meters away then there is no isolation from foreground whatsoever. Yesterday I used this lens to shoot action sports (freestyle Frisbee) in parallel with my Nikon D810 and 80-200/2.8 - the GM5 was pretty much useless for stills but a bit handy for video.
This lens seems to be a decent macro setup if used with s 10 mm extension ring - zoom lets you choose scale over a broad spectrum, and at least on the 100 mm end subject distance is away from the lens rather than inside the lens hood.
Plastic bayonet hood included.
Lumix 30 f/2.8 Macro stabilized:
Impressive resolving power, as with most macros. Focuses all the way down to 1:1 which is 17 mm width on MFT. Good background defocus, slight softness in transition from focal plane. This would be the lens to take on a nature stroll, perhaps complemented with a wide and a telezoom. No hood included - a deep screw-in hood should be mandatory.
Leica 15 f/1.7 (no stabilizer):
As expected quite a performer. With about a 30 mm equivalent field of view, it's quite versatile. Transition from focal plane to rear defocus showcases quite a bit of softness in between, which makes for beautiful close-ups. Performance wide open is great all the way to the corners, with little or no coma tendency, so it should be a great night-time city lens. Metal CNC-milled bayonet hood included, solidly made but not very effective and inside of hood reflects a bit of light. I got a slotted metal screw-in hood instead.
Lumix 25 f/1.7 (not stabilized):
Quite decent normal lens, not a stellar performer wide open but if you can find it cheap like I did ($115) then it's well worth the money. I probably won't use it so much as it's right in between the 15 and 42.5 without bringing any unique rendering skills to the table. It's also slightly larger in diameter compared to the lenses listed above so it feels a bit bulky in comparison. Plastic bayonet hood included.
GM5 is so tiny and slippery that something has not be done about that. I used gun tape on the body, as well as on the grip I got.
For me a GM5 with 2-4 lenses is a great travel kit, especially on business trip with carry-on luggage only.
It's not pocketable, but today's phones have quite decent cameras to satisfy the snapshooter need.
I would also bring a grip with dovetail plate, and a metal table-top tripod, for time lapse fun.
I tried this setup for action photography - works in a pinch but an FF DSLR with a 70-200 f/2.8 (or f/4) or a 200-500 would be massively superior.
Time lapse is fun - I'm still learning at the novice level.
Panasonic seems to know video well - not really my field of expertise.
All in all a great super-compact complement system (the body is even smaller than my old Minox).