AWS spectrum bought in 2012 is finally being put to work.
The XLTE service is the end result of Verizon's 2012 purchase of AWS spectrum. The upgrade promises better download speeds, as well as vastly improved bandwidth -- up to double in some markets. This means your down- and up-link speeds won't get slammed next time you're on the train platform, or hanging out in Downtown X.
In order to use the XLTE service, you do need to be in one of these markets at the moment (there's a lot, and more are coming). You also need to be using a newer AWS-friendly, LTE-capable device. Thankfully all the favorites -- iPhone 5s/5c, Galaxy S4/S5, HTC One M8 -- are covered (except the iPhone 4). There's a full compatible device list here.
If you're a Verizon user in one of the included markets, let us know in the comments if you're seeing better LTE speeds.