I've used Skype sparingly, mainly because of the quality. Too much delay or something, but that may have been due to the other parties' computer. I actually had a job interview over Skype, though. I'm sure 10+ years ago, this same company would have arranged for an on-site interview, but with Skype, it was a simple video conference call.
For cheap calling, I use Google Voice. It's integrated with my Sprint cell service so I just dial any number from my cell phone and if it's international, Google Voice automatically takes over and low rates (not Sprint's criminal rates) apply, or if I'm calling Canada, it's free. Plus, you can call from your computer using Gmail or the Google Voice interface. Pretty slick.