It depends. The one thing I've learned being in the wireless industry for 10 years is when it comes to service providers, you get what you pay for in terms of service and customer service. You're also going to pay full-price for the phone, which isn't cheap....a couple hundred dollars at the very least and upwards of several hundred dollars for some models, if the carrier expands it's line-up beyond the one phone it offers right now. Since it's a dedicated phone that requires special technology (UMA), you won't be able to use it with anyone else and you can't get your phone unlocked to use it on their service.
Another problem is public wi-fi....1) it's not secure, 2) signal strength depends on how close you are to the hotspot and how much traffic is on the hotspot, 3) who's providing that free wi-fi (because of reason #1).
It's just glorified pre-paid service. Which means if you break or lose your phone, it's gets stolen or damnaged, you are SOL. You'll be getting a new number and shelling out another couple hundred for a new phone. Also, if you don't refill your service by a certain date, you'll have to get a new number all over again....even if it's the next day. Once that date passes, say goodbye to your phone number. That can be a very big hastle having to get your phone number out to new people.
You're choice of phones is pretty limited. In fact, you don't have a choice right now...they only have one phone in the lineup. Their website admits that "This is Beta", meaning that you are guinea pig. There's no guarantee of service and there is no guarantee that the rate will stay at $19.99. Since it's a month-to-month agreement, they can change the terms pretty much anytime the please and you have no legal recourse since there is no contract.
In my experience and from what I see from other pay-as-you-go services (Boost, MetroPCS, Cricket, ect...), it's not worth it and most people go back to a national carrier because of the lack of customer service within a few months. The only people that stick with it long-term are those that have credit issues, can't get a regular rate plan because they lack a SS number (illegal aliens, etc.) and drug dealers / prostitutes since the accounts are untraceable.
It's not worth it if you ask me.
Correction Needed: They do offer replacement phones for a fee without having to change your number but they are refurbished units. They have 3 pricing tiers for replacement phones, so it looks like it depends on how long you are with them for.