So, I had the good idea to order the day after his announcement on a small Swiss seller and after many emails, I received them: a black HomePod Mini and a white HomePod Mini. I got US versions from a Swiss seller: the 20W chargers were in North American format.
So the installation: like any Apple product, they are borderless: I have two American models, they work perfectly with my Swiss Apple account and my French settings. I can talk to Siri and she answers me in French. The S5 processor does a gigantic job: at full volume there is no distortion, but really not, at low volume it pushes the bass for a really nice reduced sound. After that, don't expect these little balls to be used as a nightclub, they are the size of an apple, but they do a nice job. Enough to soundtrack a bedroom, I doubt for a living room size.
Where the HomePod mini already does a gigantic job solo, it gets really big when they work as a duo, paired in stereo. The sound gains in amplitude, power, richness, and I even had the impression that the bass went lower. To be seen. But the difference is impressive. So the Mini is like hamsters, take them in pairs, they will be really happy.
Configuration-wise, then it's sensitive. You should know that the HomePod connects to the same WifI as your iPhone. Always. Switch to aeroplane mode on your phone and your HomePod is all lost. Secondly, these brave beasts hate hybrid 2.4 and 5Ghz WiFi, I've experienced it. If your internet box broadcasts WiFi on two frequency bands (the two mentioned) your HomePod mini will constantly switch back and forth to the frequency band that suits it best with major inconveniences: Wifi instability, no stereo (it's direct Wifi), no Hand Off (the famous "I touch my iPhone on the HomePod and it switches music/call). In short, the HomePod likes a good WifI on a single frequency band.
Courage for those who wait for it, it's a great little speaker very well designed (Tom Holman is part of the audio staff at Apple, and Tom Holman is the T and H of THX...) with a permanent calibration frankly responsive (the famous computational audio).
Otherwise for those who expect it as an extension of their HomeKit hubs, indeed, it is the HomeKit add-on to have, to be able to increase the stability of the HomeKit home where the Apple TV was too far.
And one last tip: never unplug your HomePod if you've had it plugged in for less than 5 minutes: that's how long it takes to fully boot up and "register" on your Wifi. When I installed mine, I did a lot of quick plugs and unplugs... nothing like that to shoot up your HomeKit configuration, since they are hubs...