It will drain your battery quickly - I have an older smartwatch that will now last me a day of normal use and no more. If I turn refsix on at the start of the match with 95% battery, I'll be below 20% by half time and have to abandon it in favour of a notepad. So nowadays I don't tend to use refsix any more and instead just switch on my standard fitness tracker app as a timer - this will generally drain it to around 30% battery across a whole match.
But, when I first tried it out, my watch battery hadn't degraded as much and I used to really like it. You can type teamsheets into your phone in advance, so it will just offer you a list of names to scroll through when recording a caution - you can set it up so that instead, you type in a shirt number for cautions, which I found more awkward but you may prefer depending on the exact watch you have, so try both options. Everything is timestamped, which is great for your post-match admin (I'm terrible at taking down times of events in particular). And if you're in the middle with NAR's, you can use the app on your phone to log subs etc post-match. The phone app also has a finance tab for each match, so you can record and track your earnings across a season, which was a nice surprise that first season!
In terms of getting used to it, my suggestion would either be to practice with it while you're on a line (notwithstanding the debate we've had recently about AR's taking a match record or not), or for an even lower-pressure practice run, whenever you have a match on the TV run refsix for that match as if you're the PL ref in the middle. There are a few different settings for goals too (if you want to record goalscorer, or type of goal), so make sure you have that set up how you want in advance.
Final tip - always make sure you have a pen and blank notebook in your pocket regardless. Not particularly in case your watch fails, but more than you might find yourself needing to take notes if something particularly unusual happens, so on very rare occasions you'll still need to write by hand anyway.