I have...it works fine. I believe you have to have high speed internet (not dial up). My dear wife was calling me in the states with it, and it sounded really bad...because she was using the computer microphone (not headset) and when she plugged in a telephone, it was clear as a bell.
I also have vonage in BA - that works fine too but costs more. However, you can call UK, Italy ,Spain etc for free with vonage. Not sure with magicjack.