Andy, I have AT&T, so not a problem.
We recently got cable internet, since we are stationary for now, so I am not really in the market. However, if we had not gotten the cable internet, I would have seriously considered this. I used 30+GB last month, which luckily I had some rollover from the previous month to cover it. But WOW, still very expensive using cellular data as my only source. Our park recently added wifi, but the service is too slow for my business needs. We are very close to the 4G LTE tower, so I did have an excellent cellular connection/speed. I would have considered this product and used it as my majority of use, and scaled my ATT plan back to 15GB (15 for the price of 10).
Bill, I get those emails from FreedomPOP all the time. What I have seen is that they do not offer very much data for free. If you are very conservative with your data usage, and super-fast speed is not critical, then maybe it is a deal.
But in my graphic design business where I am sending multiple PDF proofs and emails back and forth, downloading large amounts of customer supplied images and text, uploading files to commercial printers and such, I have to have reliable, fast internet.