I'm going to try really really hard to avoid a "MagicJack is selling you a service that's almost free
" and "Skype is waaaay better suited to your needs
" discussion and point out that they have a whole forum
dedicated to just this topic.
2600 had a *great* write-up on how badly clobbered-together the MagicJack service really is back in their Winter 2009 issue.