I have a T-Mobile Sync-up that a T-Mobile reseller talked me into taking for free with a $5/month discount on the first three months of service. It's a niche product that combines the functions of a couple of different devices (OBD-II scanner, 4G hotspot w/vehicle tracking). It came in handy on a long road trip I took a couple of months ago, but around home I don't use it.
I suspect for most people it's a solution in search of a problem they don't really have, or could solve via other means. I'm going to cancel service to mine this month.