Here's the deal about Magic Jack:
Basically, for the $15 or whatever you pay, you sell over your computer and any information on your computer to their company. For Magic Jack to work, your computer has to be constantly running, and Magic Jack is constantly running background programs that eat up GOBS of memory and collect your information like any good company that makes money based on advertising. Also, the program comes with no uninstall, so getting the program off of your computer (if you want to) is near impossible. According to the wiki article I just checked, they just recently released an uninstaller for Windows that can be found on their website, but if you have a Mac, that thing is on there forever.
If you're still interested, I'd also do more research to see if it's even available where you live. A quick google showed lots of complaining customers about how there were no area codes available in their area, and apparently on top of everything else the product is nearly impossible to return during its 30-day warranty.