For me it works better than a regular cell phone, because it works in my basement workshop where I can't get reception from most carriers.
They use some heuristic to determine when you're using the cellular network "too much", because their whole pitch is that traffic over a cellular network is more expensive than traffic over the existing ISPs. If you use their cellular network too much, they will terminate your contract - but again, I'm not a customer so I don't know where that line lives.
Also, I believe while on a call you can transfer off of wifi, but not on to it.
Here's the most recent statement on that matter, I think: http://republicwireless.com/blog/unlimited
(Also, I think they made the wrong call, and they should have "stepped back from unlimited". It won't be unlimited, they shouldn't call it unlimited. But I'm not planning on abusing it either way.)
I'm sitting in their Wave G, patiently waiting. I refuse to pay for both a nice home connection and a cell data connection, so I still have a dumbphone. Buying a smart phone that primarily uses local Wifi (which I am in about 90% of the time) but can use cellular networks as a backup when there's no Wifi, and thus only charges for that "backup" contingency, is something I'm willing to pay for. I'll recover the cost of purchasing the phone in something like 6 months, even ignoring the fact it'll be a much better phone in the meantime.