If you have the right situation, it's awesome
This is great once you find the reason to use it! Skype alone is nothing and has no pedagogical merit, you have to create the situation to make it a powerful tool. Find another classroom, just type "Skype Pen Pals" into google, and talk to them through Skype before exchanging letters. When a staff member goes on vacation or if an ex-staff member goes to China, give them a call with your students. If you find an interesting person in a field your students are studying, send them an e-mail and see if they'll jump on Skype with you! This is a great "world expanding" and "background knowledge building" tool, once you create the use case for it. Any critique I would give Skype, not helping more in finding "Skype pals" or not connecting you to experts automatically is null and void once you visit https://education.skype.com/ where they will help you engage your classroom from the bottom up. One gripe, which isn't on Skype, is that you do need proper bandwidth to make this work, choppy calls are engagement killers for students, you can either try and get your school to upgrade your bandwidth, plug in your computer hardline, or try just a voice call with no video (as a last resort, since the video is super engaging). Also plan your "Skype dates" with other teachers, so the whole school isn't making video calls at the same time.
How I Use It
I have used skype to engage students with pen-pals and to connect with people across the world. I've used it to connect my student to cool people in the world, to other classrooms, and also to staff that have traveled.