I have had Sprint for the last 8 years and have their unlimited data from day one. Started with a BlackBerry, feathered it to my laptop, used an insane amount of data, never a word from sprint.
For the last 3 years I have been on my epic 4g running custom ROMs with hotspot hacked, with 2 laptops and 2 kindle fires tethered almost every night, and still not a word from sprint about data usage.
I couldn't be happier with my service and signal at my house. I also use the Nav, Pandora, Netflix constantly without issue...