Everyone has their favorites, but after looking at the Superguide, I took my iPhone to Airvoice using their
$10/month plan (they also use AT&T towers). If you use mostly iMessage, that just uses wifi data if you're connected, or a tiny amount of data if you're on cellular data. Regular texts are .02 each, voice calls are .04/minute, and data is .066/MB. We have VoIP at home, so I probably only use my cell for a couple of calls and texts a month (not counting iMessage). I use a good bit of data, but the remainder carries over. Right now I have a $30 surplus (handy when we go on vacation).
If you don't have VoIP and need to make a lot of cell calls, they have
unlimited talk/text plans as well. No insurance, no tax, limited hours for support (but they're based in the US, which is nice).
One thing I found is that AT&T inflated the amount of data I used compared to what Airvoice does, but turning off all auto updates and such really cuts down on data.
If you go off AT&T, though, you will lose visual voicemail. I don't know how other MVNO carriers handle it voicemail notifications, but I get no notice at all if someone leaves a message. Of course you do get a missed call notification (assuming you're not in a dead zone), but I do miss that little red circle.