I have had skype for going on 2 years, costs me $60 a year with a added phone number so my family in Chicago can call me as a local rather then long distance to my home number, and I have yet to have one single problem at all with it. I use a Linsys portable phone I got off ebay that really makes using skype even easier.
I have never tried magic jack yet, but not really wanting to just because skype is so cheap as it is compared to what it would cost me per month with all I get with skype, I dont feel the need to change a good thing atm.
I should mention I have yet to have a problem with the above stated, only though when I was on a 6mb line, and was downloading games to gametap, at which point the internet line would slow down, but this was not a problem with skype, but bandwith being used. if I tried to use it, I would get garbled sound, but if not downloading games when I was on a 6mb line, it never gave any other problems. 
NOW though I am on a 10mb line it doesnt matter if I am downloading game to game tap, or doing what ever the hell I want on the internet, the only time it messes up is when I am over working my pc and try to make a call, thats a big no no, and yet again not a skype problem, but my slow ass single core pc.