Thought about it before. If I were single and didn't use my phone much, maybe. We have 2 lines, 1GB of shared data, all the features and minutes I want for $88+tax. It makes more sense to go the other way and simply hotspot my phone data to the iPad. $70 for 2 iPads and then still have to pay for a number somewhere (e.g. Skype) so kids/family can get a hold of us works out to be similar. Like I said, I thought about it but I was paying over $120 for the same 2 lines back then so the math was a little different. We also don't have a home phone so my wife uses her iPhone quite a bit as a result.
It doesn't appear that it would work out too well for us in particular, even if I consider getting a home phone. But with the right scenario, I could see it working for some.
If you're not stuck on a contract, you could always try it out for a month or two. Worst case, you could always go back to using your iPhone.
"You think that's air you're breathing?"