I have been using FaceTime for about 3 years now. The best part about using FaceTime is that it is Apple to Apple only. What that means is you would get a good camera everytime, and thus the quality of the call would be great.
FaceTime does require a slightly higher bandwidth compared to other video calling apps, but it compensates for it by providing uninterrupted, clean and enjoyable experience.
The sound quality to and fro is quite good. you do not need to stay logged in like Skype to receive a call. Let it stay off, but as soon as you get a call, the app would start by itself and request you to accept the call. That for me is pure genius. Also, if you have am iPhone and Mac, this app allows you to make phone calls from your iPhone through the mac. This means you can have your iPhone lying around in the room, and still make regular phone calls from your mac.
It is far better than Skype, in terms of call quality, service, and price.