I've got one. It's a pain in the ass, but it serves it's function when travelling.
It doesn't have a lot of functionality, and you are unable to block your number when making an outgoing call, and frequently when receiving an incoming call, it blows out the magicjack completely.
I used to use it mainly when I was on conference calls and my lines would be tied up or three-wayed between 2 different conference bridges, it allowed me to make outbound calls without disconnecting a conference. It's also good for making outbound calls leaving your regular phone line free for incoming, ...but only when it works. It frequently doesn't. It's cheap I will admit, ...but it's very wonky and unpredictable. So far, it has blown out 2 of my phones, and customer support is less than desireable. You cannot talk to anyone, customer support is all chat based. It serves up constant flash advertising in your face, and when it switches between adds, it interferes with the clarity of the call. Cheap, but not worth the money.
It's great for Europeans, Africans, or Asians etc who need to make frequent calls into US & Canada.