Our Millenicom/Verizon service has been solid from the time we first started using it back in the 3G only days until right now with their 4G hotspot. There have been some issues at times, I understand, with Verizon throttling the Millenicom customers, just as they do with their own direct customers under certain usage situations, but Millenicom has worked hard at getting that cleared up, since Millenicom customers are not supposed to be throttled. Millenicom was able to restore full speed service to a number of new customers at one point, by assigning them to legacy account numbers that were not affected by Verizon's throttling policy. I wouldn't be surprised though, to find that they've run out of those accounts. All in all, Millenicom has done a good job for us pretty much anywhere we've been. We do see the occasional congested tower of course, just as all cell customers do at times.
As for a Verizon store clerk not knowing about Millenicom, I'm not surprised at all, since it's not a product they sell. Heck, it's hard to find a clerk that's really knowledgeable about the products they DO sell! How can Millenicom sell the service so cheap? Easy, they buy bandwidth in bulk from Verizon at a price that allows them to resell it cheaper than Verizon can because they don't have all the infrastructure overhead to maintain that Verizon does. Why does Verizon sell them bulk data that cheaply? Because they make as much or more profit from bulk sales as they do from individual customers, since they don't have the accounting and other overhead costs that individual direct customers incur. Millenicom is an "MVNO" (Mobile Virtual Network Operator), no different than the other no contract month to month providers like the multiple Tracfone brands, etc. They buy at wholesale prices and resell at discounted retail prices. It's called "free enterprise".