No device is perfect. Magic Jack's international long distance does not reach every country on earth. There is a list on the back of the package of the countries that can be reached with this plug-in. Double check to be sure the countries that you may be calling are covered by using Magic Jack before you purchase.
Another issue is that the internet needs to be on at all times, thereby using more electricity. Of course, machines do go to "sleep," but with a recent emphasis on conserving energy, keeping a computer on again can cost. Shutting off the computer only affects incoming calls, so the middle of the night be all right. Otherwise, you will just have to leave the internet on at all times.
The final concern is that one USB port needs to be dedicated to the constant use of the Magic Jack plug-in. Again, this is a small issue since most computers have extra ports anyway.