I have one. Works fine. Sometimes the audio quality is about like a cell phone, but other times it sounds like a land line. I also use Skype, and both are comparable. I was able to just 'borrow' the modular-jacked room telephone from timeshares in Mexico and Portugal, take them to the place where we had free wi-fi, plug the phone into the MagicJack and dial away.
Can't say that I'd use it for either a fax line or as a sub for regular phone service. We have kids/grandkids in a distant city, so I put our MagicJack's number there so their calls to us are to a local number (for them) and of course no charge to us on the MJ.