I have a “Series 0”. I’ve found that Activity sync has gotten extremely slow in iOS 13. It sometimes takes almost an entire day for the data to show up on my phone. Right now, at 17:33, it’s only showing data until about 11 AM, so presumably it hasn’t synced for about six hours. It does eventually catch up.
Also, battery life on the Watch seems to have gone way down. It does feel like this is a case of it attempting to push data to the phone, failing, and trying yet again, this wasting CPU cycles (and therefore battery), but I don’t know this for a fact. I can’t upgrade the Series 0 to anything newer than watchOS 4.3.2 so I don’t know if 6.x helps (which some have suggested in threads here). [This is possibly fixed in iOS beta 8!]
No issues otherwise. It does… basically work. I’ll probably get a newer model soon anyway.