So I am fortunate enough to have had the following setup: 1 HomePod in a master bedroom, 1 HomePod in an open concept kitchen, 2 stereo HomePods in a cathedral ceiling Family Room. Now I moved the HomePod from the master bedroom to a basement fitness room and added a stereo pair of 2 HomePod minis in the master bedroom, and a HomePod mini in a small home office.
My thoughts? First off let the excitement of sound quality for the minis be realistic, once played at any medium volume or higher they get CRUSHED by the main HomePod which not only has far more bass but sounds so much richer and nuanced across the whole music range. Putting the minis in a stereo pair helped a lot to make them sound richer but the bass still obviously is lacking but for my bedroom where I only play music at a low volume and don't WANT a lot of bass they make a ton of sense and the stereo sound has been a nice upgrade. The mini was also appropriate for my home office at it sits right on the desk in front of me for podcasts and quiet music. Overall I find the mini to be a brighter nicer sound than a 3rd generation Echo (I don't have the 4th generation ball Echo to compare to) and a very nice sounding device (and looking, it looks great) at its price. I would NOT use the mini in a large open room like in many homes (family rooms, kitchens) unless you are using them primarily for voice, or "background" music at a lower volume.
PS: I'm almost most excited by moving that full sized HomePod to my basement fitness room as now I'm really set for Fitness+ on a wall TV with the HomePod blasting below it. Even not in stereo with the hardwood floor it really gets the volume and bass sound ripping which is great for spin bike / treadmill workouts etc.