Iridium has some advantages. It has coverage at the poles, where GEO sats typically do not. The mobile units can be small(ish) and handheld. But it also has some significant limitations. Datarates are in the 2400bps range. Latency goes all over the place due to the way the calls get routed through the constellation. Dropped calls are common.
Inmarsat operates a service called BGAN (Broadband Global Area Network), which depending on your hardware delivers speeds in the 128kbps to 512kbps range. The latency is always bad (GEO is a long way away), but more consistent. The terminals are big bulky affairs that range in size from a briefcase up to a mini-fridge. You can't hang one on your belt like you can with an Iridium phone, and some require you to set the antenna up and point it at the satellite manually.
While it is true that the Iridium company bought the system for a song, it's also true that they bought a massively expensive maintenance liability. You can't just ignore a satellite and expect it to keep working, they require operators on the ground to regularly monitor each and every bird to insure that it doesn't drift off orbit and to handle conditions that arise. They also have to launch replacement satellites regularly as the old ones start to fail. Plus they're building out a whole new system. The ROI is no doubt positive (they've been doing this for years now), but maybe not as much as you might expect. Iridium's biggest problem was its tiny userbase.
Motorola's whole business model with the original Iridium was pretty insane. They saw the relatively sparse deployment of cell towers back in the AMP era as something that was going to last. The only way to fix it was satellite communications, but the phones needed to be small enough to be used like cell phones (admittedly, compared to a 1980s cell phone they really weren't too bulky), which means low power which means LEO, which means you need a ton of satellites to cover the globe. By the time they finally got them all launched cell towers were everywhere and people (especially businessmen) realized that they liked using them indoors (which Iridium was terrible at) and paying only pennies per minute (instead of dollars per minute). The target audience was small, and many of those people couldn't afford the system at all.
