This may be of help to those considering Mint. I just got back from a long road trip using Mint, and am pretty pleased with the service (I am on the 12-month, 10 GB plan). Drove from Atlanta to Pittsburgh to Cleveland and back to Atlanta over the past week, and Mint provided very good service for the most part. I did a few speed tests while travelling, and was able to record my fastest 4g speed ever at around 48 mbps near Cleveland. It was great to be able to use my phone as a mobile hotspot in the car so I could use my laptop (this is something my old carrier, Cricket, didn't provide).
The only real downer was that there was a lengthy chunk of road in Virginia/West Virginia during which i got no service at all (I later saw that Mint provides no coverage there at all). Fortunately I had downloaded offline Google Maps for those areas so navigation wasn't affected, and I was able to connect to the internet using my wife's phone as a mobile hotspot (she has Verizon prepaid, which did have service the whole time, but her's costs around $20 more per month than my plan). Also, at a stop I was able to make a call by logging in to a restaurant wifi and using WiFi Talk.
Overall, there are some limitations to Mint but the advantages and value are hard to beat.