o you can turn off automatic top-off (you wont get charged $10).
o if you ALLOW the $10, it will allow for overages (I think 500mb-1gb -- cant recall)
o you can BYOD. My son has an iphone 5c on it. With "freedompop friends", he gets an additional 500MB free. Thats (200min/500text/1gb data free)
For $7.99 you get 2g outgoing calls (uses cell service rather than VOIP), voice mail, data rollover and a few other services. I had that for about 2 years on my spare phone (hell of a lot cheaper than regular service). It's a good deal.
They also have their "global sim". 200min/500text/200MB free across most of europe and the US. "freedompop friends" can bump that up to 700MB. It cost me $1.99 for the sim -- I turned off all the "pay services" before I would get my first bill and dropped the sim in my tablet. More data than my 200mb for life tmobile sim. Plus it's service is in Great Britain -- (goes through a provider in London) I can access BBC videos in California reserved for GB only. Latency is high (low 200-mid-to-upper 300) but speed is very good. Latency also improves over a constant connection (telnet, for example).
That said, God help you if you ever need to call customer service.