Magic Jack: Too Good To Be True?

So, I got one of these Magic Jack things from a friend. And though it seems like hoodoo, I think we may try it. Here's an excerpt from the PC Magazine review:

Those commercials advertising cheap voice-over-IP (VoIP) phone rates look appealing, but there's always some fine print: You have to switch cable-TV providers, install a VoIP-smart Internet router, or something. And you end up ticked once a month when the bill from MCI, Verizon, or some other telecom robber baron dips into your wallet. If this sounds like your bio, the YMax magicJack is for you: 2 minutes of setup, voice quality that's almost indistinguishable from a straight analog POTS line, and a cost of just under $20 a year for unlimited nationwide service. ... MagicJack's call quality is amazing—almost too good to be true.