Ok here are my observations. I have changed my watch ever since Series 0 back in 2015. I wasn't going to upgrade from my S5 this year because the differences were too slight and I never had any issues with my S5 and reading some of the issues on this forum, it seems that a lot of people are having challenges of one sort or another with the S6/WatchOS7. But i got offered such a good price that i sold it and got the 6.
My watch arrived today, and it paired up with no problem, 10 mins to transfer the LTE data plan from my S5 to the S6 and 15 minutes to update to 7.01. Unpaired and re-paired straight after the OS update and then switched off my phone, then switched off my watch, powered the phone back up and the powered the watch back up This is a routine that i always do and it seems to work for me after an update.
Having done all that, I have spent my first day wearing the watch and my observations are that the screen is definitely brighter outdoors and unless its my imagination, it seems a little brighter indoors. The S6 processor seems faster than the S5 processor as apps load faster, Siri seems to respond quicker. The AOD definitely responds quicker. In fact everything seems a little bit brisker and smoother. Blood Oxygen app works well without the bands being too tight. The ECG function app still works (I understand that some guys have had some loss of ECG functionality with the S6). Finally the battery life seem just as good as my S5. Loss of 3% an hour. For me it was well worth the upgrade so one happy customer.