In a word, excellent. I had a play with the timer and some other settings pre game and it worked almost perfectly. Goal, subs and cautions were all easy to record and post match the upload and subsequent match history/time-line in the app was great. Only fault I could find was it didn't pick up or register HR at all but that may be more down to my wearing of the watch rather than the app. I'll have to trial that out a bit more in future games.
I note above that it can be linked to a HR strap, I may look to do that if the watch feature continues to not work, or be inaccurate.
I can see it will have its limitations on a Sunday morning when I'm out on my own with poor/badly written/late teamsheets etc but at a semi-pro or decent level where there is three officials on a 'proper' game it will be excellent. I'm aiming to go as paperless as possible and RefSix on the Apple Watch will deffo help in that. For the five quid a month they charge I'll definitely be subscribing for the remainder of this season and then will look to get the annual one from next season onwards.
Battery life was great, no issues there. Only slight interruption was from other apps (Whatsapp messages mainly) popping up during the first half. A quick mute of various things solved that for the second with no further interruptions. Heat-map looked pretty accurate when compared to what my Garmin 610 GPS said I'd been doing. There are a few things that will take a game or two to get used to.....stopping the timer during the game and at end of halves but all very minor. After just two games I think I'm very much a convert. Give it a go, I don't think you'll be disappointed.