Got my HomePod and I was very interested in using it with an Apple TV, here are some notes to anyone considering it:
When the Apple TV is connected, you can issue play/pause/skip forward/skip back commands to the HomePod handsfree and it will work on the Apple TV. I haven't figured out all of these commands yet - I just discovered you can say "Hey siri, skip forward 45 seconds" and it works. I'm sure there are a bunch.
For anything other than Music and Videos (Games for example) the sound will get GRATUITOUSLY out of sync. By up to 5 seconds for me. Videos are delayed by the software so that they are in sync with the audio - this can't be done for games.
The volume when playing videos on the Apple TV is not particularly loud, even when completely maxed out. Music is much much louder than videos. I have to turn the volume down significantly if I start playing a song after I've been watching video.
Forwarding sound from the Apple TV sounds a bit strange as there's no stereo separation. Going from the crappy built in TV speakers to the HomePod is almost a downgrade, but there's obviously a lot more bass. It sounds truly mono this way. This doesn't apply when playing music through the Apple TV, only video content.
Also in iTunes when you're playing something on the Apple TV through the HomePod it will group the two devices together, and anything you play via iTunes will play on the Apple TV, which is fantastic.
HOWEVER, if you issue a voice command to play music through the HomePod (NOT for any other request, it won't disconnect if you ask it the weather for example) it will disconnect from the Apple TV and become its own device again which means you'll have to reconnect it to use it with the ATV again. I think maybe Apple could forward HomePod requests to Siri on the Apple TV in the future which would fix this problem and keep them merged as one device at all times, but I don't know for certain. This would have the added benefit of being able to use the Apple TV remote to control playback on the HomePod, even if the TV isn't necessarily turned on, as well as issuing Apple TV-specific commands with onscreen visual feedback handsfree.