I say this in a lot of threads, but you can get genuinely unlimited data on ATT towers from Cricket for ~$60/month. It is limited to 8Mbps for LTE and 4Mbps for HSPA+, which isn't bad. Practically, I get about 400kb/s download speed on the slower network or 800kb/s LTE download speed. I have used over 100GB every month for the past year or so and have never have a problem. They do throttle people who use a lot of data, but only if there is network congestion and you have used more than 22GB in the month. Even in cities I don't get throttled all that often.
I know a lot of people advocate the grandfathered plans you can get from ebay (there was even a seminar about it at the RTR). While undoubtedly a screaming deal and probably okay, the downside of any of those is that you are working outside the TOS of the cell provider. Generally someone is selling you an old plan that they own with grandfathered in rates/terms. The problem is that ATT (or whoever the provider is) can shut down these plans without any notice at all.
As with many things in life it depends on your risk tolerance. I tend to side with what Riverman said above about never paying an individual and only doing business directly with the company or through an authorized reseller.