Air Voice gsm has been around for a long time relatively speaking. At least since the early 2000s. They are a lot older then say Simple Mobile or a few other mvnos. They have identical coverage to the Att Gophone prepaid network. Their strong points are US customer service, lowest paygo rates of any Att mvno (slightly), and offer the lowest paygo data of any Att mvno (but still high at $2 a meg).
Their data offering is the weakest of all the major Att mvnos . They have 30 megs/100 mms vs say H20 or Red Pocket's 50 megs/unlimited mms at the same price ($50 or so for unlimited talk/text plus mms/data).
Other Att mvnos have better international rates by far (H20, Red Pocket and Jolt Gsm all beat Air Voice in that area).
Pure Talk is another Att mvno contender but they have ZERO data though they are supposedly working on it. They also offer strong intl voice/text plans.
Air Voice is okay. They also have a dollar a month e911 fee plus they charge 20 cents for the first minute and regular 10 cents a minute after that.
Page Plus has better coverage and a strong bring your own phone system along with better paygo rates.
Their unlimited plan is cheaper then Air Voice by five dollars and offers only 10 megs less.
Page Plus has been around since the 90s as a paging service and as a prepaid wireless carrier since the early 2000s. They have been the center of some controversy but they are pretty stable.
Air Voice is okay. Nothing spectacular but they do offer a solid lineup. Just that they are somewhat outclassed by their other Att mvno sisters in certain areas. Especially data/mms/intl calling.
I wouldn't be afraid of using them.