I’ll write here some impressions after using the Garmin 945 for a few days and some running sessions.
So far, the general feeling is positive, with some highlights:
- the watch is light and pleasant to wear all time (not that I had issues previously with the Fenix line, even over night)
- the vibration is a bit stronger than Fenix (maybe the size, maybe the materials .. but it feels stronger than Fenix)
- the screen contrast is better than Fenix 5X+ (most probably is the sapphire on the Fenix compared with the Gorilla DX glass on 945 – I noticed that when I moved from Fenix 3 to Fenix 5)
- the backlight is powerful (even at 20%), I’d say a bit more powerful than Fenix (but maybe it’s just the glass as mentioned above)
- the race prediction is more accurate (I’d say) because it takes into account not only the VO2 max value, but also the load and the trend of your recent training. Till now, I always felt like Garmin is too optimistic (encouraging, sure .. ). Some details are available here https://www.firstbeat.com/en/consumer-feature/race-time-predictor/.
From the GPS accuracy, it’s not better or worse than the Fenix 5X+, the track doesn’t look awful (if you’re interested just to understand where you were), but obviously, for those people looking for best tracks – they will be disappointed. I should mention the fact that I run in a city with no skyscrapers or long tree covered tracks.
Garmin is working on some improvements of the GPS (at least that’s what they say on the 2.50 release notes, that some GPS improvements will come later) – so maybe we will see that in the next firmware releases.
Regarding the daily use, I see no difference from Fenix, the oHR seems decent, but I haven’t had any complaints before.
On the negative side .. the sturdiness of Fenix and maybe the size are those things that I miss now, but that will go with time.
Liviu Nastasa 
I’ve run with Garmin for about 10 years now and am on my 3rd watch, a 610.
The 945 looks huge! My watch doesn’t have the race predictor. That looks cool but it could be annoying.
When I’m running or racing I only look at pace, elapsed time and total distance.
Actually, in terms of size and weight, the 945 is just a bit larger than the 610 (47mm vs 45.7mm) but lighter and thinner – it may be that my picture is not reflecting the size properly.
For me, coming from Fenix 5X+ (51mm), I see that the 945 is smaller and I still try to appreciate that.
In terms of screens used when running, I also use the regular distance/pace screen and during the training at the HR values; the race predictor is only used (eventually) after the training sessions as a nice data gimmick based on the athlete evolution. What I see with 945 is the fact that it takes more data into consideration when doing the estimation and now is closer to being trustworthy. With the Fenix estimation, I always discarded it .. since it was overly-optimistic and not realistic, not even for half marathon, not to mention marathon.