My sister-in-law lives in Spain and my wife and I call her landline phone there for 2¢ a minute using Skype which is much less than dialing direct.
We've called her Skype-to-Skype for free, but often get an echo and the connection is not as good. So we cough up a whopping 2 cents a minute and call her landline phone, and I've been very pleased with the service. I only rarely use the IM feature.
Once in a blue moon--you can guess how old I am with that expression--we'll use up our 700 monthly "anytime" cell phone minutes so we use our Skpye phone to call US numbers. Again, it's only 2¢/min which is much better than 45¢/min. cell phone overage charges. The only disadvantage being that you have to call from your computer.
If you're going to use Skype, I recommend headphones with a mic or a USB handset which I have. You'll get much better sound than from your computer microphone and speakers.
The Skype user forums have lots of complaints about the Skypein online number and since I don't see any need for an online number, I've been reluctant to get one. Otherwise, it's a great service.