well, I do not have this product, but do a quick google check with the words:
majic jack complaint
There is a whole lot of people complaining about it, everything from slowing down, connection issues, no real tech support, no way to return the item, no phone number to call, instead of the 30 days trial, they actually charge you immediately.....list goes on.
Also, there is positives about it too, so I guess, its up to you to decide which side to choose. No product is ever perfect for everyone.
As from the posts above me, all of them seems to have quite a good experiences from it, there are posts online from people buying it 2009 and complaining about it, so go figure.