I just signed up for Virgin Mobile. I pay 55 a month for unlimited calling, data and texting. After we really see how many minutes I am talking we may change to a cheaper plan with limits on calling. I think you can automatically pay if you want. I just pay monthly. I get a reminder text and pay when I get it. The service seems to be great. I haven't had any problems with dropped calls, roaming or spotty service, while my old AT&T didn't work well in my house at all.
I did have to buy a Virgin mobile phone. I was told they couldn't or wouldn't use other brand phones. They don't offer the tremendous phone discounts other carriers do, but I got an older model smart phone that I am extremely pleased with and didn't mind paying for...the math definitely works out in the end.
I was also able to port over my digital phone, so I was able to keep my business number. That took some time (a little over a week), mostly because of the digital carrier, not Virgin. But Virgin did get it taken care of and called me regularly to update me on the status of the switch. Their customer service all center was obviously in India, but they did a great job of things.
I am new to it, but so far, I have been extremely happy.