People reviewing it so far haven't done any meaningful reviews. On the other hand, some people are waiting for music magazines putting a HomePod into a lab and measuring things, and I can only tell you that this will not give any meaningful results. Like they will try to measure the frequency response. At the same time, the HomePod will measure the layout of their lab, and produce the best result it can in that lab room. It will twist its frequency response so that the ears of the listener get the best possible result, and if you look at the measurements, they will look godawful. Because you have a speaker that doesn't try to produce the best sound coming out of the speaker, but the best sound entering the listener's ears.
It seems to me that if the A8 processing is all it’s cracked up to be, the HomePod should actually be able to give a relatively flat frequency response. Otherwise Apple will have done a half-assed job imo. Most wireless speakers like this can’t ‘hear’ themselves, but the HomePod can because it has microphones. This feature is actually relatively common in the home theater arena, with room correction software built into the receiver like Audyssey, YPAO, Dirac, etc. When you first set up the receiver, you plug a microphone into it and it plays test tones and figures out how to compensate for both the speakers you’re using and the room itself. Apple’s processing should, at least in theory, perform the same type of thing.