Got MagicJack recently. Voice quality is much better than the old VoIP service I used. Almost never get telemarketer calls on it (unlike my cell). Lessons learned so far:

1. It seems to work best on faster cable systems and faster computers. Slower computers may mean a delay getting a line when you call out.
2. If you can connect the computer directly to your router it seems to work better. (The PC I use is connected by WiFi to the router).
3. Several mornings the phone started ringing contiuously but no caller ID and no answer when I picked up. (Could be automatic Windows Updates or something related to the wireless phone setup).

As these systems make our home networks much more complex, I now make it a point to reset my cable modem and Magic Jack PC every day to renew the "DCHP lease" .