I was going to switch to Fi a while ago but I got spooked by the data plans. i wound up getting Mint Mobile - $25 a month with unlimited everything but there is a data cap of 12 g a month. I'm a relatively medium data user (love my YouTube for music videos during the day) but I never go over the cap. Yes, you have to prepay - but I was happy to pay $300 for a years service! I've used it in Northeastern Ohio and on a drive from Fort Lauderdale to Atlanta and never had an issue.
They have a special running now if you can live with 8 gig a month - 6 months service for $60.