Nice. Which parent company did you get the plan from?
I am among the few who bought Sprint's $15 Kickstart plan during the two weeks it was available last year. It's not as nice as yours—there is a 50GB cap before deprioritization (not throttling [1]) occurs, and I presume yours allows for hotspot—but an unlimited plan for $15 is amazing, even if on the Sprint network.
I'd heard that Sprint is good about honoring grandfathered plans. Your experience further encourages me that $15 Kickstart will continue through the T-Mobile acquisition.