My mailbox is in a block of other mailboxes that is fairly far from my home (say, 500m) and out of wifi range without an especially large antenna. I was thinking of making a small battery-powered widget to tell me when the mail arrived and adding it to my Home Assistant (https://www.home-assistant.io/) installation.

Another use may be something like a device that warns if a locked, remote, storage area is accessed. In fact, almost any alarm system that lacks a cellular backup could add one pretty cheaply with this.

Now that I know this exists, I'm going to use it as a bag tracker to tell me where my baggage went when I travel. I'll probably just get one and use it as my car tracker to tell me when to move for street parking. Or maybe I can just inactive it.

https://www.eatabit.io is built on it. We were one of their first beta clients. We used every M2M carrier previously and Twilio is easily superior in cost & reliability. The cellular data space has alot of problems especially with roaming: cell phones transparently switch between networks but data SIMs have a very limited set of networks they can operate on. If Twilio can get VZW access on these SIMs they will have a very strong product. As it stands, coverage is limited.