Who uses it, and what for? I can imagine a few uses for emergency comms in particularly out-of-the-way locations, but it's hard to imagine how that makes enough money to maintain an entire constellation of satellites.
Transmitter is very small as well. https://www.iridium.com/products/details/iridium-9603
The current constellation got bought for pennies on the dollar when Iridium went bankrupt. I believe this new generation is the first new constellation they've put up since then. It will be interesting to see if they can make it work. Launch costs are a lot lower now, which ought to help a lot.
Per https://en.wikipedia.org/wiki/Iridium_satellite_constellatio... the service is useful for journalists, explorers and military units.