How I Use It
I have used Skype to connect with other classrooms (our sister school classrooms), with experts (in all sorts of subjects), and with students who are home sick.
It's pretty easy as long as there aren't any wifi/bandwidth issues at my school. You do need a camera--I used to use the camera on my laptop but that doesn't as easily give the speaker a view of the class. I ended buying a cheap USB web camera that I mount on top of my board so that the speaker can see the students (you can easily get a usb "extension cord" from Amazon or tech retailers.) If you want the students to interact easily with the speaker, a cheap karaoke mic that plugs into your microphone/headphone jack works well. Otherwise, just have a student or two speak into your computer.
One thing to keep in mind are time zones and communicating clearly about the time you are connecting. There are been at least two instances were the speaker and I agreed on a time but didn't do a good job of communicating which time zone we were each in.
I use Skype to connect with experts and other classes around the world. When we are studying a topic, I reach out to my friends, family, and social network for people who are willing to talk to my students about the topic and/or their job related to the topic. I love when they can actually come into the classroom, but that limits the people I can reach out. Skype allows me to have people talk to my students, regardless of where they are located. People are also much more willing to do a quick 20-30 minute conversation than commit to leave their job, driving to my school, and then talking to my students.