I just used Apple’s iMessage via satellite for the first time in the Yukon. My carrier is Verizon, so iMessage handled the satellite messaging. Overall, it was outstanding compared to my Garmin inReach. It worked reliably outdoors under clear skies, with some tree cover, and even from inside my KUIU Mountain Star tent. The only thing I still needed the inReach for was weather. I’ll keep the Garmin as a backup until iMessage matures and adds weather reports.
For me, battery life wasn’t an issue with the iPhone. I keep it off when not in use, run in Low Power Mode, in airplane mode when I don't need to communicate and dim the screen—since the display is the biggest power draw. I’m not constantly checking my phone, so I don’t mind when messages download after I power it on. I also don’t mind pointing the phone at the satellite or waiting a bit to send. My longest wait was about 12 minutes, but most connections were immediate or within a minute or two. For the long one, I set a 12-minute timer, did other tasks, and used the phone when the connection was ready.