Had a chance to play with HomePod yesterday. Some random impressions:
It is really small. I knew it was going to be small from reading reviews, but it is even smaller that I thought. It is about the size of a cantaloupe.
It feels like a bit of a beta product. We had a bunch of Airplay drop outs when we navigated away from the music play apps on our phones. Neither of us had Apple Music, but the iOS Tidal and Spotify apps worked fine over Airplay except for the occasional dropouts. I imagine a lot of this will get fixed with updates.
This isn't replacing anyone's two-channel system. As a smart speaker, it is impressive and possibly the best sounding smart speaker I've heard. That being said, we tested it in a larger room and I thought it struggled. It is solid for what it is, but there is only so much it can do in a decent sized room.
Apple really needed to have stereo functionality with two HomePods at launch. A bunch of reviewers mentioned that Apple demoed the stereo functionality for them and that it sounded great. I believe them and I think having a second HomePod in a stereo configuration might help a lot. One HomePod sounds big for a mono speaker, but it still sounds like a mono speaker.
Overall, I'd say I'm lukewarm about HomePod. It is impressive for what it is, but don't go expecting a $350 smart speaker to work miracles. There is only so much it can do sonically in a moderate/large room, even with whatever DSP tricks Apple has worked into it. I get the impression that a lot of the reviewers who gushed about the sound quality don't know what a proper stereo rig can sound like.
EDIT: Looks like Apple's staged tests for the media that the HomePod easily won against the other smart speakers might not stand up: http://pogueman.tumblr.com/post/170722337727/head-to-head-does-the-apple-homepod-really-sound