Republic Wireless?

Does anyone have experience with Republic Wireless? My grandparents have been using two 8 year old nokia candybar phones from AT&T for a while now and are looking to upgrade. They are currently paying AT&T $60/month for two lines with only unlimited calling. Last year they purchased a Samsung galaxy tab 7 and love it. So I was thinking of getting them both S4's as it would be familiar. But AT&T's pricing is through the roof (as we discovered shortly after deciding to go to their store)...

Long story short, Republic Wireless has the best deal by far. $10 a month for unlimited talk and text is an amazing deal, and even after the cost of the devices, they'll be saving money after about 10 months.

But there has to be drawbacks, right? What's it like switching from wifi to cellular while you're on a call? How good is Sprint's coverage (I'm in SLC, Utah)? Any problems with the Moto G on their service?