I installed it last night; like everyone else, it took some time, about 90 minutes to 2 hours (I didn't time it.)
The few things (randomly) that I have noticed so far:
- The large heartrate complication no longer shows your last reading - it just says tap to get a reading. I'm not sure why Apple made this change; I'll likely just take the complication off now, as it's pretty useless as a complication in a large slot.
- Text size on workout data fields is quite a bit smaller and harder to read. I think that was also the wrong direction to go.
- I am trying out the Siri watch face again, and it is definitely more useful this time. With watchOS 4 I gave up on it after less than a day; this time I'll try it a while longer.
- I wish there was a way to uninstall the walk-talkie app. I'll never use it. (I know that I can uninstall it indirectly by uninstalling FaceTime from the iPhone, but I'd prefer not to go that far. I rarely use FT, but some people do make audio calls to me that way.)
- This is not watchOS 5 specific, but, boy do I wish there were more digital watch faces that show seconds other than the Activity digital watch face.
- Thumbs-up to allowing third party audio apps to have volume control and easy media transfer. For the first time I may go out for a run without the iPhone to see how well that will work. (I still prefer having the phone with me for emergencies, though. I run often in very rural areas with spotty LTE and GSM coverage, so I'm not sure a cellular watch is a good enough alternative yet.)
- Thumbs-up as well for allowing edit of the control center.
It'll probably take a few more days of use to get reaction to everything new.