I've been beta testing the ios 18 satellite messaging since mid-summer.. Its been fairly reliable and seems to work as advertised -- and now that ios 18 is out in the wild, I can message pretty much anyone else with an updated IOS device now.
Occasionally it will not go into satellite mode even though I have no apparent cell or wifi signal though. I've found it useful to communicate briefly "hey, updated ETA will be 3:55pm". Drawbacks is that you can't send regular TXT messages to non-IOS devices, sending the messages takes fairly long (about 5-7 seconds), and you have to keep your phone aimed at a certain point (which slowly moves or sometimes changes to a completely different direction as another satellite takes over). Advantage is that you already have the device, its free, and its transparent (aside from a message appearing buddy's iMessage app).. but there's no "weird" phone number or shortcode that your messages will be sent from.
SOS feature is still there, as is the "Update my position via satellite" in the Find My app. Its a good feature to add to your toolbag in case of a wilderness crash/unplanned landing. With an old 121.5Mhz ELT, you'll be lucky if anyone hears it even in populated areas; with a 406Mhz ELT you'll be located within about 5km; and with a GPS-connected 406Mhz ELT, its within a tenth of a minute ( 43⁰ 34.1') [about a 1/10th of a mile].. but you have no idea for sure if anyone received your distress call until hours later. At least with the iPhone, you can have two-way communication and can know for sure that help is on the way, along with more specific coordinates.